fix(basemap): serve CARTO key from backend, bound the map gate, add source attribution

Review follow-up:

- Drop the Next.js route. CARTO_API_KEY is now a regular backend registry
  key (env, .env, or the API Keys panel) served by public
  GET /api/basemap-config. Every frontend mode already proxies /api/* to
  the backend (Next.js proxy in web mode, companion server in packaged
  desktop), so this covers web and desktop with one mechanism and leaves
  the static export untouched. Also removes the invalid non-handler
  export from the route module by removing the module.
- useBasemapConfig: fail open to the unkeyed style after 3 s, abort the
  request at 15 s, apply a late key when it arrives, cache successes per
  page and retry failures on the next mount.
- Declare OSM/CARTO attribution on the raster source (same markup as the
  viewer's existing AttributionControl so MapLibre de-duplicates it).
- Tests: backend endpoint (unset / set+trimmed / persisted operator key /
  registry), hook behaviour (success, non-OK, network error, soft timeout
  then late key, hard abort, shared request and retry), attribution and
  gating source checks.
- CARTO_API_KEY moves to the backend service in docker-compose.yml; docs
  updated accordingly.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

backend/main.py

@@ -9072,7 +9072,7 @@ async def api_sentinel_tile(request: Request):
 # ---------------------------------------------------------------------------
 # API Settings — key registry & management
 # ---------------------------------------------------------------------------
-from services.api_settings import get_api_keys, get_env_path_info
+from services.api_settings import get_api_keys, get_basemap_config, get_env_path_info
 from services.shodan_connector import (
     ShodanConnectorError,
     count_shodan,
@@ -9111,6 +9111,12 @@ async def api_get_keys_meta(request: Request):
     return get_env_path_info()
 
 
+@app.get("/api/basemap-config")
+@limiter.limit("60/minute")
+async def api_basemap_config(request: Request):
+    return get_basemap_config()
+
+
 @app.get("/api/tools/shodan/status", dependencies=[Depends(require_local_operator)])
 @limiter.limit("30/minute")
 async def api_shodan_status(request: Request):