feat: database stats on the dashboard via GET /api/stats

New SQLAdapter.get_database_stats() aggregates totals, station count,
date range, and hourly-slot coverage in one query per dialect. The
endpoint follows the existing 503-guard/to_thread/TTL-cache pattern;
the dashboard gains a five-tile stats strip on the existing refresh
cadence. Coverage denominator is hour-truncated so off-hour endpoints
cannot push it past 100%; MySQL slot expression avoids % characters
that would break under pyformat bind interpolation.

src/web_api.py

@@ -51,6 +51,10 @@ FORECAST_CACHE: Dict[str, tuple] = {}
 FORECAST_CACHE_LOCK = Lock()
 FORECAST_TTL = 900  # 15 minutes
 
+DB_STATS_CACHE: Dict[str, tuple] = {}
+DB_STATS_CACHE_LOCK = Lock()
+DB_STATS_TTL = 300  # 5 minutes
+
 # Admin API protection. Read/dashboard endpoints stay public; anything that
 # mutates state or leaks configuration requires the X-API-Key header matching
 # ADMIN_API_KEY. Secure by default: with no key configured, those endpoints
@@ -630,6 +634,45 @@ async def get_station_measurements(
         raise HTTPException(status_code=500, detail=str(e))
 
 
+@app.get("/api/stats")
+async def get_database_stats():
+    """Get database coverage statistics (totals, date range, hourly coverage)"""
+    increment_counter("api_requests", labels={"endpoint": "api_stats"})
+
+    scraper = app_state["scraper"]
+    if not scraper or not scraper.db_adapter:
+        raise HTTPException(status_code=503, detail="Database not available")
+
+    now = time.monotonic()
+    with DB_STATS_CACHE_LOCK:
+        cached = DB_STATS_CACHE.get("all")
+        if cached and now - cached[0] < DB_STATS_TTL:
+            return cached[1]
+
+    try:
+        stats = await asyncio.to_thread(scraper.db_adapter.get_database_stats)
+    except Exception as e:
+        logger.error(f"Error fetching database stats: {e}")
+        raise HTTPException(status_code=500, detail=str(e))
+
+    if stats is None:
+        raise HTTPException(status_code=503, detail="Database statistics unavailable")
+
+    first_ts = stats["first_timestamp"]
+    last_ts = stats["last_timestamp"]
+    data = {
+        "total_measurements": stats["total_measurements"],
+        "station_count": stats["station_count"],
+        "first_timestamp": first_ts.isoformat(),
+        "last_timestamp": last_ts.isoformat(),
+        "days_spanned": (last_ts.date() - first_ts.date()).days + 1,
+        "coverage_percent": stats["coverage_percent"],
+    }
+    with DB_STATS_CACHE_LOCK:
+        DB_STATS_CACHE["all"] = (now, data)
+    return data
+
+
 @app.post("/scrape/trigger", dependencies=[Depends(require_admin_key)])
 async def trigger_scraping(background_tasks: BackgroundTasks):
     """Trigger manual data scraping"""