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.

src/web_api.py

@@ -353,8 +353,9 @@ async def create_station(station: StationCreateModel):
         existing_ids = [int(sid) for sid in scraper.station_mapping.keys()]
         new_station_id = max(existing_ids) + 1 if existing_ids else 1
 
-        # Add to station mapping
-        scraper.station_mapping[str(new_station_id)] = {
+        # Add to station mapping and persist
+        new_key = str(new_station_id)
+        scraper.station_mapping[new_key] = {
             "code": station.station_code,
             "thai_name": station.thai_name,
             "english_name": station.english_name,
@@ -362,6 +363,9 @@ async def create_station(station: StationCreateModel):
             "longitude": station.longitude,
             "geohash": station.geohash,
         }
+        if not scraper.save_stations():
+            scraper.station_mapping.pop(new_key, None)
+            raise HTTPException(status_code=500, detail="Failed to persist new station")
 
         logger.info(f"Created new station: {station.station_code} ({station.english_name})")
 
@@ -376,6 +380,8 @@ async def create_station(station: StationCreateModel):
             status=station.status,
         )
 
+    except HTTPException:
+        raise
     except Exception as e:
         logger.error(f"Error creating station: {e}")
         raise HTTPException(status_code=500, detail=str(e))
@@ -396,6 +402,7 @@ async def update_station(station_id: int, updates: StationUpdateModel):
 
     try:
         station_info = scraper.station_mapping[station_key]
+        original = dict(station_info)  # snapshot for rollback if persistence fails
 
         # Update fields if provided
         if updates.thai_name is not None:
@@ -409,6 +416,10 @@ async def update_station(station_id: int, updates: StationUpdateModel):
         if updates.geohash is not None:
             station_info["geohash"] = updates.geohash
 
+        if not scraper.save_stations():
+            scraper.station_mapping[station_key] = original
+            raise HTTPException(status_code=500, detail="Failed to persist station update")
+
         logger.info(f"Updated station {station_id}: {station_info['code']}")
 
         return StationResponse(
@@ -422,6 +433,8 @@ async def update_station(station_id: int, updates: StationUpdateModel):
             status=updates.status or "active",
         )
 
+    except HTTPException:
+        raise
     except Exception as e:
         logger.error(f"Error updating station {station_id}: {e}")
         raise HTTPException(status_code=500, detail=str(e))
@@ -442,10 +455,17 @@ async def delete_station(station_id: int):
 
     try:
         station_info = scraper.station_mapping.pop(station_key)
+
+        if not scraper.save_stations():
+            scraper.station_mapping[station_key] = station_info  # restore
+            raise HTTPException(status_code=500, detail="Failed to persist station deletion")
+
         logger.info(f"Deleted station {station_id}: {station_info['code']}")
 
         return {"message": f"Station {station_info['code']} deleted successfully"}
 
+    except HTTPException:
+        raise
     except Exception as e:
         logger.error(f"Error deleting station {station_id}: {e}")
         raise HTTPException(status_code=500, detail=str(e))