Public overpass-api.de throttled later heavy query-types (429/504) →
only ~1099/3000 utility elements loaded. Harden the loader:
- per-query retry-with-backoff on 429/5xx (esp. 504): up to 3 attempts
per endpoint, exponential backoff (10s/30s/60s) + jitter, honoring the
Retry-After header when present.
- mirror fallback: OVERPASS_ENDPOINTS list (overpass-api.de →
kumi.systems → private.coffee); exhausting one endpoint rotates to the
next mirror.
- inter-query delay bumped 1s → 3s (gentler on the public endpoint).
- a query that fails all retries+mirrors is logged and skipped
(partial-load, not abort) — existing per-element SAVEPOINT/UPSERT kept.
Tests: 429-then-200 retry, persistent-429 primary → mirror fallback,
504 → mirror, all-fail skip-without-abort, one-failing-query-does-not-
abort-others, plus Retry-After/backoff helper units. All mocked, no
real Overpass.
