Prevent duplicate Nominatim requests during proximity searches

[andreagrandi]

Context

The map and list panels request proximity results concurrently. On a forward-geocode cache miss, both requests could contact Nominatim, while the default geopy adapter could also retry HTTP 429 responses immediately. This increased upstream traffic and delayed the existing keyword fallback.

What changed

• Normalize and hash place/country lookup identities into backend-safe cache keys.
• Use the shared Django database cache as a short-lived single-flight lock so concurrent callers reuse one geocoding result.
• Cache unresolved and failed lookups for 60 seconds while retaining the existing six-hour successful-result cache.
• Disable geopy transport retries so one elected lookup produces one upstream HTTP attempt.
• Add focused coverage for normalized cache reuse, concurrent callers, retry configuration, and rate-limited failures.
• Add a Chromium E2E that submits a proximity filter through the live Django server, overlaps the map/list requests, and verifies a single geocoding operation with consistent responses.

Notes / constraints

• Existing map/list proximity responses and keyword fallback behavior are preserved.
• Reverse geocoding is unchanged.
• No API schema, database migration, dependency, template, or documentation changes are required.

Testing

• nox -s tests — passed.
• nox -s e2e -- tests/e2e/test_map.py::test_proximity_filter_deduplicates_concurrent_map_and_list_geocoding — full browser suite passed.
• git diff --cached --check — passed before commit.

Closes #177