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/data/stations.json

@@ -0,0 +1,130 @@
+{
+  "1": {
+    "code": "P.20",
+    "thai_name": "บ้านเชียงดาว",
+    "english_name": "Ban Chiang Dao",
+    "latitude": 19.36731448032191,
+    "longitude": 98.9688487015384,
+    "geohash": null
+  },
+  "2": {
+    "code": "P.75",
+    "thai_name": "บ้านช่อแล",
+    "english_name": "Ban Chai Lat",
+    "latitude": 19.145972935976225,
+    "longitude": 99.00735727149247,
+    "geohash": null
+  },
+  "3": {
+    "code": "P.92",
+    "thai_name": "บ้านเมืองกึ๊ด",
+    "english_name": "Ban Muang Aut",
+    "latitude": 19.220518985435646,
+    "longitude": 98.84733127007874,
+    "geohash": null
+  },
+  "4": {
+    "code": "P.4A",
+    "thai_name": "บ้านแม่แตง",
+    "english_name": "Ban Mae Taeng",
+    "latitude": 19.1222679952378,
+    "longitude": 98.94437462084075,
+    "geohash": null
+  },
+  "5": {
+    "code": "P.67",
+    "thai_name": "บ้านแม่แต",
+    "english_name": "Ban Tae",
+    "latitude": 19.009762080002453,
+    "longitude": 98.95978297135508,
+    "geohash": null
+  },
+  "6": {
+    "code": "P.21",
+    "thai_name": "บ้านริมใต้",
+    "english_name": "Ban Rim Tai",
+    "latitude": 18.917459157963293,
+    "longitude": 98.97018092996231,
+    "geohash": null
+  },
+  "7": {
+    "code": "P.103",
+    "thai_name": "สะพานวงแหวนรอบ 3",
+    "english_name": "Ring Bridge 3",
+    "latitude": 18.86664807441675,
+    "longitude": 98.9781107622432,
+    "geohash": null
+  },
+  "8": {
+    "code": "P.1",
+    "thai_name": "สะพานนวรัฐ",
+    "english_name": "Nawarat Bridge",
+    "latitude": 18.7875,
+    "longitude": 99.0045,
+    "geohash": "w5q6uuhvfcfp25"
+  },
+  "9": {
+    "code": "P.82",
+    "thai_name": "บ้านสบวิน",
+    "english_name": "Ban Sob win",
+    "latitude": 18.6519444,
+    "longitude": 98.69,
+    "geohash": null
+  },
+  "10": {
+    "code": "P.84",
+    "thai_name": "บ้านพันตน",
+    "english_name": "Ban Panton",
+    "latitude": 18.591315274591334,
+    "longitude": 98.79657058508496,
+    "geohash": null
+  },
+  "11": {
+    "code": "P.81",
+    "thai_name": "บ้านโป่ง",
+    "english_name": "Ban Pong",
+    "latitude": 13.805661820610888,
+    "longitude": 99.87174946122846,
+    "geohash": null
+  },
+  "12": {
+    "code": "P.5",
+    "thai_name": "สะพานท่านาง",
+    "english_name": "Tha Nang Bridge",
+    "latitude": 18.580269437546555,
+    "longitude": 99.01021397084362,
+    "geohash": null
+  },
+  "13": {
+    "code": "P.77",
+    "thai_name": "บ้านสบแม่สะป๊วด",
+    "english_name": "Baan Sop Mae Sapuord",
+    "latitude": 18.433347475179602,
+    "longitude": 99.08510036666527,
+    "geohash": null
+  },
+  "14": {
+    "code": "P.87",
+    "thai_name": "บ้านป่าซาง",
+    "english_name": "Ban Pa Sang",
+    "latitude": 18.519121825282486,
+    "longitude": 98.94224374138238,
+    "geohash": null
+  },
+  "15": {
+    "code": "P.76",
+    "thai_name": "บ้านแม่อีไฮ",
+    "english_name": "Banb Mae I Hai",
+    "latitude": 18.141465831254404,
+    "longitude": 98.89642508267181,
+    "geohash": null
+  },
+  "16": {
+    "code": "P.85",
+    "thai_name": "บ้านหล่ายแก้ว",
+    "english_name": "Baan Lai Kaew",
+    "latitude": 18.17856361002219,
+    "longitude": 98.63023114782287,
+    "geohash": null
+  }
+}