Persist station CRUD across restarts via JSON config

Station CRUD via the API previously mutated the scraper's in-memory
station_mapping only, so changes were lost on restart (and the systemd
service auto-restarts).

- Extract the 130-line hardcoded station_mapping into bundled defaults at
  src/data/stations.json; the scraper loads from a runtime-writable config
  file (STATION_CONFIG_PATH, default stations.json) and falls back to the
  bundled defaults to seed it.
- Add scraper.save_stations() with an atomic temp-file + os.replace write.
- create/update/delete station endpoints now persist and roll back the
  in-memory change if the write fails; re-raise HTTPException so persistence
  errors surface as real 500s instead of being swallowed.
- Backend-agnostic (works for the VictoriaMetrics deployment, which has no
  relational stations table). Runtime stations.json is gitignored.

Also clears pre-existing flake8 debt in water_scraper_v3.py (unused imports,
long lines, duplicate logging import) and dedupes the User-Agent to
Config.USER_AGENT.

.gitignore

@@ -134,4 +134,6 @@ cython_debug/
 
 # Docker volumes
 vm_data/
-grafana_data/
+grafana_data/
+# Runtime station config (persisted CRUD); bundled default lives in src/data/
+/stations.json