[NREM][Space] Add Nominatim geocoding cache and rate-limit protection for multi-widget pages

[numbers-official]

Summary

The reverseGeocode function in asset-service.ts calls OpenStreetMap Nominatim on every fetchAsset invocation when assetLocationCreated is absent. On pages embedding multiple <capture-eye> widgets, this can trigger multiple concurrent Nominatim requests, risking rate-limit violations under Nominatim Usage Policy (max 1 req/sec, no bulk automated queries).

Evidence

Identified during PR #93 review (conversation fe90808c, 2026-03-19). The reviewer explicitly flagged:

如果同一頁面有多個 Capture Eye widget 同時開啟，可能會觸發 rate limit。

While a 3-second AbortController timeout was added to prevent indefinite blocking, there is no request deduplication or rate-limiting mechanism.

Current Behavior

Widget A opens → fetchAsset → reverseGeocode(lat1, lon1) → Nominatim
Widget B opens → fetchAsset → reverseGeocode(lat2, lon2) → Nominatim  (concurrent)
Widget C opens → fetchAsset → reverseGeocode(lat3, lon3) → Nominatim  (concurrent)

All requests fire simultaneously with no coordination.

Proposed Approach

1. In-memory geocoding cache: Cache (lat, lon) → address results in a module-level Map so repeated coordinates skip the API call entirely
2. Request queue with 1 req/sec throttle: Serialize Nominatim requests with a minimum 1-second interval between calls
3. Coordinate rounding: Round lat/lon to ~4 decimal places (~11m accuracy) before cache lookup to increase cache hit rate

Impact

• Without fix: Pages with 5+ capture-eye widgets will likely hit Nominatim rate limits, causing geocoding failures and falling back to raw coordinates
• With fix: Compliant with Nominatim usage policy, better UX from cached results, reduced external API dependency

Related

• Issue [Feature] Add in-memory asset data cache to eliminate redundant API calls on repeated modal opens #78 (in-memory asset data cache) — similar caching pattern but for asset API, not geocoding
• PR [Omni] Fix Capture Eye widget failing to load data on open #93 — where the reverseGeocode feature was introduced

Generated by NREM Mode with Omni