diff --git a/tradein-mvp/backend/app/core/config.py b/tradein-mvp/backend/app/core/config.py
index 3b5ffbb7..dcf7b42c 100644
--- a/tradein-mvp/backend/app/core/config.py
+++ b/tradein-mvp/backend/app/core/config.py
@@ -6,14 +6,17 @@ from pydantic_settings import BaseSettings, SettingsConfigDict
 class Settings(BaseSettings):
     model_config = SettingsConfigDict(env_file=".env", env_file_encoding="utf-8", extra="ignore")
 
-    database_url: str = "postgresql+psycopg://tradein:tradein@postgres:5432/tradein"
+    # required — задаётся через env DATABASE_URL. Нет дефолта: fail-fast при старте
+    # если переменная не задана (C-3 security audit).
+    database_url: str
     cors_origins: list[str] = ["http://localhost", "http://localhost:3000", "http://localhost:8080"]
     environment: str = "dev"
 
     # Geocoder
     yandex_geocoder_key: str | None = None        # 25K req/day free после регистрации
     yandex_suggest_key: str | None = None         # для frontend autocomplete (proxy через backend)
-    contact_email: str = "erginrajpopxbe@outlook.com"  # для User-Agent в Nominatim (Nominatim Usage Policy)
+    # для User-Agent в Nominatim (Nominatim Usage Policy)
+    contact_email: str = "erginrajpopxbe@outlook.com"
 
     # Public URL — для QR-кода в PDF, shareable links, etc.
     public_url: str = "http://127.0.0.1:8080"
diff --git a/tradein-mvp/backend/app/services/estimator.py b/tradein-mvp/backend/app/services/estimator.py
index f4f81cdb..7dfd2ff3 100644
--- a/tradein-mvp/backend/app/services/estimator.py
+++ b/tradein-mvp/backend/app/services/estimator.py
@@ -23,7 +23,7 @@ import json
 import logging
 from datetime import UTC, datetime, timedelta
 from typing import Any
-from uuid import UUID, uuid4
+from uuid import uuid4
 
 from sqlalchemy import text
 from sqlalchemy.orm import Session
@@ -83,7 +83,7 @@ async def estimate_quality(
     if geo is None:
         # Без координат не можем искать через PostGIS. Возвращаем low confidence.
         logger.warning("geocode failed for %s — returning low-confidence estimate", payload.address)
-        return _empty_estimate(payload, reason="address_not_geocoded")
+        return _empty_estimate(payload, db, reason="address_not_geocoded")
 
     # 2. #392: обогащаем год / тип дома из картографии (OSM Overpass), если
     #    пользователь их не указал — это улучшает house-match аналогов (#6).
@@ -562,19 +562,79 @@ def _deal_to_analog(row: dict[str, Any]) -> AnalogLot:
     )
 
 
-def _empty_estimate(payload: TradeInEstimateInput, *, reason: str) -> AggregatedEstimate:
-    """Fallback когда нет данных для оценки."""
+def _empty_estimate(
+    payload: TradeInEstimateInput, db: Session, *, reason: str
+) -> AggregatedEstimate:
+    """Fallback когда нет данных для оценки.
+
+    Сохраняет запись в БД (confidence='low', пустые analogs/deals), чтобы GET /estimate/{id}
+    не возвращал 404. C-4 security audit.
+    """
+    estimate_id = uuid4()
     now = datetime.now(tz=UTC)
+    expires_at = now + timedelta(hours=24)
+
+    db.execute(
+        text(
+            """
+            INSERT INTO trade_in_estimates (
+                id, address,
+                area_m2, rooms, floor, total_floors,
+                year_built, house_type, repair_state, has_balcony,
+                ownership_type, has_mortgage, client_name, client_phone,
+                median_price, range_low, range_high, median_price_per_m2,
+                confidence, confidence_explanation, n_analogs,
+                analogs, actual_deals,
+                sources_used,
+                expires_at
+            ) VALUES (
+                CAST(:id AS uuid), :address,
+                :area, :rooms, :floor, :total_floors,
+                :year_built, :house_type, :repair_state, :has_balcony,
+                :ownership_type, :has_mortgage, :client_name, :client_phone,
+                0, 0, 0, 0,
+                'low', :explanation, 0,
+                '[]'::jsonb, '[]'::jsonb,
+                '[]'::jsonb,
+                :expires_at
+            )
+            """
+        ),
+        {
+            "id": str(estimate_id),
+            "address": payload.address,
+            "area": payload.area_m2,
+            "rooms": payload.rooms,
+            "floor": payload.floor,
+            "total_floors": payload.total_floors,
+            "year_built": payload.year_built,
+            "house_type": payload.house_type,
+            "repair_state": payload.repair_state,
+            "has_balcony": payload.has_balcony,
+            "ownership_type": payload.ownership_type,
+            "has_mortgage": payload.has_mortgage,
+            "client_name": payload.client_name,
+            "client_phone": payload.client_phone,
+            "explanation": reason,
+            "expires_at": expires_at,
+        },
+    )
+    db.commit()
+    logger.info(
+        "empty_estimate: id=%s reason=%s addr=%s", estimate_id, reason, payload.address[:60]
+    )
+
     return AggregatedEstimate(
-        estimate_id=uuid4(),
+        estimate_id=estimate_id,
         median_price_rub=0,
         range_low_rub=0,
         range_high_rub=0,
         median_price_per_m2=0,
         confidence="low",
+        confidence_explanation=reason,
         n_analogs=0,
         period_months=DEALS_PERIOD_MONTHS,
         analogs=[],
         actual_deals=[],
-        expires_at=now + timedelta(hours=24),
+        expires_at=expires_at,
     )
diff --git a/tradein-mvp/backend/app/services/exporters/trade_in_pdf.py b/tradein-mvp/backend/app/services/exporters/trade_in_pdf.py
index d68f68c9..bec1f4b7 100644
--- a/tradein-mvp/backend/app/services/exporters/trade_in_pdf.py
+++ b/tradein-mvp/backend/app/services/exporters/trade_in_pdf.py
@@ -17,6 +17,7 @@ import datetime as dt
 import html as _html
 import io
 import logging
+from urllib.parse import urlparse
 from uuid import UUID
 
 import segno
@@ -45,6 +46,46 @@ def _bar_svg_data_url(q_low: float = 0.20, q_high: float = 0.80) -> str:
 logger = logging.getLogger(__name__)
 
 
+# ── URL allowlist (C-6 security audit) ─────────────────────────────────────
+# Только доверенные домены-источники объявлений попадают в <a href=> в PDF.
+# Защита от javascript: / data: инъекций через source_url объявлений.
+_ALLOWED_URL_DOMAINS: frozenset[str] = frozenset({
+    "www.avito.ru", "avito.ru", "m.avito.ru",
+    "www.cian.ru", "cian.ru",
+    "realty.yandex.ru",
+    "n1.ru", "ekaterinburg.n1.ru",
+})
+
+# CDN-домены для изображений аналогов (photo_url).
+_ALLOWED_PHOTO_CDN: frozenset[str] = frozenset({
+    "images.avito.st",
+    "cdn-p.cian.site",
+    "avatars.mds.yandex.net",
+    "n1ru.cdn-cw.com",
+})
+
+
+def _safe_url(url: str | None, *, cdn: bool = False) -> str | None:
+    """Проверяет URL по allowlist схемы и домена.
+
+    cdn=False — allowlist для source_url объявлений (ссылки на листинги).
+    cdn=True  — allowlist для photo_url (CDN изображений).
+    Возвращает None если URL не прошёл проверку — embed/ссылка пропускается.
+    """
+    if not url:
+        return None
+    try:
+        p = urlparse(url)
+    except Exception:
+        return None
+    if p.scheme != "https":
+        return None
+    allowed = _ALLOWED_PHOTO_CDN if cdn else _ALLOWED_URL_DOMAINS
+    if p.netloc not in allowed:
+        return None
+    return url
+
+
 # ── Source pseudo-logos (текстовые pill-badges как у Брусники с логотипами) ──
 _SOURCE_LOGO_COLORS: dict[str, tuple[str, str]] = {
     "avito":     ("#00aaff", "#fff"),    # Avito brand blue (упрощённо)
@@ -484,8 +525,9 @@ def _examples_rows(lots: list[AnalogLot]) -> str:
     rows = []
     for lot in lots:
         addr = _html.escape(lot.address)
-        if lot.source_url:
-            addr_cell = f"<a href='{_html.escape(lot.source_url)}' style='color:#1d4ed8;text-decoration:none;'>{addr} ↗</a>"
+        safe = _safe_url(lot.source_url)
+        if safe:
+            addr_cell = f"<a href='{_html.escape(safe)}' style='color:#1d4ed8;text-decoration:none;'>{addr} ↗</a>"
         else:
             addr_cell = addr
         rows.append(