fix(maps): make Overpass endpoints configurable and harden the POI search (#1309)

Builds on @Hardik-369's instance-specific User-Agent idea and reworks the rest
of the #1309 fix:

- keep the unique User-Agent (buildUserAgent) — a shared UA gets the public
  Overpass mirrors to rate-limit harder; it appends the configured instance
  URL and is applied to every Nominatim/Overpass/Wikimedia call
- add OVERPASS_URL so an operator behind locked-down egress (e.g. a Kubernetes
  cluster) can point the explore search at an internal/self-hosted Overpass
  instance instead of the public mirrors
- keep the per-endpoint timeout default at 12s but make it tunable via
  OVERPASS_TIMEOUT_MS for slow self-hosted instances; non-positive/invalid
  values fall back to the default rather than 502-ing every search at a 0ms cap
- log each endpoint's failure reason before the 502 so blocked egress is
  diagnosable instead of a bare "Overpass request failed"

Adds unit tests for the User-Agent, endpoint and timeout resolution plus the
all-mirrors-down path, and documents the two new env vars in .env.example, the
wiki and the Helm chart.

charts/trek/values.yaml

@@ -67,6 +67,12 @@ env:
   # Max MCP API requests per user per minute. Defaults to 300.
   # MCP_MAX_SESSION_PER_USER: "20"
   # Max concurrent MCP sessions per user. Defaults to 20.
+  # OVERPASS_URL: ""
+  # Custom Overpass endpoint(s) for the map POI "explore" search, comma-separated. When set, REPLACES the bundled
+  # public mirrors — point it at an internal/self-hosted Overpass instance when the public mirrors are unreachable
+  # from the cluster (e.g. locked-down egress). Non-http(s) entries are ignored.
+  # OVERPASS_TIMEOUT_MS: "12000"
+  # Per-endpoint timeout (ms) for Overpass POI requests. Raise it for a slow self-hosted Overpass instance. Defaults to 12000.
 
 
 # Secret environment variables stored in a Kubernetes Secret.