fix: BUG-13 — server geocode returns raw coords instead of location names

Root cause: server _geocode_cached() returned None → /api/geocode
returned HTTP 204 with no body. JS frontend treated 204 as success,
r.json() on empty body threw → showed raw-coordinate fallback.

Server fixes:
- _geocode_cached() now always returns a non-empty string (coordinate
  fallback when both Nominatim and reverse_geocoder fail)
- /api/geocode always returns HTTP 200 with {label: '...'} (never 204)
- /api/geocode/batch always includes every coord key in result dict
- Nominatim call: jsonv2 + accept-language=en (aligned with Pi)
- reverse_geocoder: mode=1 (aligned with Pi)

Pi alignment:
- _geocode_cached() now checks for empty reverse_geocoder label before
  returning, falling through to coordinate string (same guard as server)

BUGS.md

@@ -9,16 +9,25 @@ When CLOSED, move the entire entry to Resolved — don't duplicate.
 ## Open bugs
 
 ### BUG-13 — Server Web UI shows raw coordinates instead of geocoded location on trips
-- **Status**: `OPEN`
+- **Status**: `IN PROGRESS`
 - **Reported**: 2026-08-12
 - **Affected trips**: 18517, 18516 (and possibly others)
 - **Symptom**: The trip detail/replay page shows raw (x, y) coordinates instead of
   reverse-geocoded location names (street/city/address).
-- **Likely cause**: Geocoding pipeline is either not running, failing silently, or
-  the cache lookup isn't populating for these coordinates. Could be a regression
-  from the BUG-09 geocode preloading changes or a server-side cache issue.
-- **Plan**: Rework the location/geocoding pipeline primarily on the server side.
-  If simple enough, also backport the fix to the Pi (local dash) for consistency.
+- **Root cause**: Server `_geocode_cached()` returned `None` when both Nominatim and
+  reverse_geocoder failed to produce a label. The `/api/geocode` endpoint then returned
+  HTTP 204 with no body. The frontend JS treated 204 as success, called `r.json()` on
+  empty body → threw → showed raw-coordinate fallback. The Pi version always returned
+  a label string (with coordinate fallback) and never 204.
+- **Fix**: 
+  - Server `_geocode_cached()` now always returns a non-empty string — coordinate
+    fallback (`lat, lon`) when both geocoders fail, matching Pi behavior.
+  - Server `/api/geocode` always returns HTTP 200 with `{label: "..."}` (never 204).
+  - Server `/api/geocode/batch` always includes every coord key in the result dict.
+  - Server Nominatim call now uses `jsonv2` + `accept-language=en` (aligned with Pi).
+  - Server reverse_geocoder now uses `mode=1` (aligned with Pi).
+  - Pi `_geocode_cached()` now checks for empty reverse_geocoder label before
+    returning (same guard the server has), falling through to coordinate string.
 
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