feat: --fill-gaps all repairs missing data across the whole DB range

Gap detection is now hour-granular: days with partial data (missing
hourly slots) are re-fetched, not just days with no rows at all. The
API's hour-24-is-next-midnight quirk is handled by re-fetching day D-1
when day D is missing its 00:00 slot. SQL adapters gain single-query
range and recorded-hours lookups; non-SQL backends fall back to the
old day-granular check.

src/main.py

@@ -180,9 +180,12 @@ def run_continuous_monitoring():
     return True
 
 
-def run_gap_filling(days_back: int):
-    """Run gap filling for missing data"""
-    logger.info(f"Checking for data gaps in the last {days_back} days...")
+def run_gap_filling(days_back: Optional[int]):
+    """Run gap filling for missing data (days_back=None scans the whole range)"""
+    if days_back is None:
+        logger.info("Checking for data gaps across the whole data range...")
+    else:
+        logger.info(f"Checking for data gaps in the last {days_back} days...")
 
     try:
         # Validate configuration
@@ -445,6 +448,7 @@ Examples:
   %(prog)s                          # Run continuous monitoring
   %(prog)s --web-api                # Start web API server
   %(prog)s --fill-gaps 7            # Fill missing data for last 7 days
+  %(prog)s --fill-gaps all          # Fill missing data across the whole data range
   %(prog)s --update-data 2          # Update existing data for last 2 days
   %(prog)s --import-historical 2024-01-01 2024-01-31  # Import historical data
   %(prog)s --status                 # Show system status
@@ -461,9 +465,11 @@ Examples:
 
     parser.add_argument(
         "--fill-gaps",
-        type=int,
-        metavar="DAYS",
-        help="Fill missing data gaps for the specified number of days back",
+        metavar="DAYS|all",
+        help=(
+            "Fill missing data gaps for the specified number of days back, "
+            "or 'all' to scan the entire data range in the database"
+        ),
     )
 
     parser.add_argument(
@@ -529,7 +535,17 @@ Examples:
         elif args.web_api:
             success = run_web_api()
         elif args.fill_gaps is not None:
-            success = run_gap_filling(args.fill_gaps)
+            if args.fill_gaps.lower() == "all":
+                success = run_gap_filling(None)
+            else:
+                try:
+                    success = run_gap_filling(int(args.fill_gaps))
+                except ValueError:
+                    logger.error(
+                        f"Invalid --fill-gaps value '{args.fill_gaps}': "
+                        "expected a number of days or 'all'"
+                    )
+                    sys.exit(1)
         elif args.update_data is not None:
             success = run_data_update(args.update_data)
         elif args.import_historical is not None: