Extract API Pydantic models into schemas.py

Move the seven request/response models out of web_api.py into a dedicated
src/schemas.py (separation of concerns; first step of the file-size cleanup).
web_api.py imports them back, so behaviour is unchanged.

Note: web_api.py is still over the 500-line guideline; the remaining bulk is
the inline HTML dashboard in root(), to be extracted in a follow-up.

src/schemas.py

@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+"""Pydantic request/response schemas for the water monitoring web API."""
+
+from datetime import datetime
+from typing import Any, Dict, Optional
+
+from pydantic import BaseModel, Field
+
+
+class StationResponse(BaseModel):
+    station_id: int
+    station_code: str
+    thai_name: str
+    english_name: str
+    latitude: Optional[float] = None
+    longitude: Optional[float] = None
+    geohash: Optional[str] = None
+    status: str = "active"
+
+
+class StationCreateModel(BaseModel):
+    station_code: str = Field(..., description="Station code (e.g., P.1, P.20)")
+    thai_name: str = Field(..., description="Thai name of the station")
+    english_name: str = Field(..., description="English name of the station")
+    latitude: Optional[float] = Field(None, ge=-90, le=90, description="Latitude coordinate")
+    longitude: Optional[float] = Field(None, ge=-180, le=180, description="Longitude coordinate")
+    geohash: Optional[str] = Field(None, description="Geohash for the location")
+    status: str = Field("active", description="Station status")
+
+
+class StationUpdateModel(BaseModel):
+    thai_name: Optional[str] = Field(None, description="Thai name of the station")
+    english_name: Optional[str] = Field(None, description="English name of the station")
+    latitude: Optional[float] = Field(None, ge=-90, le=90, description="Latitude coordinate")
+    longitude: Optional[float] = Field(None, ge=-180, le=180, description="Longitude coordinate")
+    geohash: Optional[str] = Field(None, description="Geohash for the location")
+    status: Optional[str] = Field(None, description="Station status")
+
+
+class MeasurementResponse(BaseModel):
+    timestamp: datetime
+    station_code: str
+    station_name_en: str
+    station_name_th: str
+    water_level: float
+    discharge: Optional[float] = None
+    discharge_percent: Optional[float] = None
+    status: str = "active"
+
+
+class HealthResponse(BaseModel):
+    overall_status: str
+    timestamp: str
+    checks: Dict[str, Dict[str, Any]]
+
+
+class MetricsResponse(BaseModel):
+    counters: Dict[str, float]
+    gauges: Dict[str, float]
+    histograms: Dict[str, Dict[str, float]]
+
+
+class ScrapingStatusResponse(BaseModel):
+    is_running: bool
+    last_run: Optional[datetime] = None
+    next_run: Optional[datetime] = None
+    total_runs: int = 0
+    successful_runs: int = 0
+    failed_runs: int = 0