docs: add BUG-13 — server web UI shows raw coords instead of geocoded location

- Trips 18517, 18516 show (x,y) instead of address/street name
- Likely regression in geocoding pipeline or cache lookup
- Plan: rework server-side geocoding, backport to Pi if simple

BUGS.md

@@ -8,7 +8,17 @@ When CLOSED, move the entire entry to Resolved — don't duplicate.
 
 ## Open bugs
 
-(No open bugs — all Phase 26 issues resolved.)
+### BUG-13 — Server Web UI shows raw coordinates instead of geocoded location on trips
+- **Status**: `OPEN`
+- **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.
 
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