feat: add historical backfill with --init CLI and episode numbering

Adds a --init mode that seeds the database with past shows from a given
anchor episode/date forward, batch-fetching likes from SoundCloud and
partitioning them into weekly buckets. Episode numbers are tracked in
the shows table and auto-incremented by the poller for new shows.

Includes full API documentation (docs/api.md) and updated README.

Made-with: Cursor

src/ntr_fetcher/main.py

@@ -1,9 +1,12 @@
+import argparse
 import asyncio
 import logging
+from datetime import date
 
 import uvicorn
 
 from ntr_fetcher.api import create_app
+from ntr_fetcher.backfill import run_backfill
 from ntr_fetcher.config import Settings
 from ntr_fetcher.db import Database
 from ntr_fetcher.poller import Poller
@@ -16,13 +19,51 @@ logging.basicConfig(
 logger = logging.getLogger(__name__)
 
 
+def _parse_args() -> argparse.Namespace:
+    parser = argparse.ArgumentParser(description="NtR SoundCloud Fetcher")
+    parser.add_argument(
+        "--init", action="store_true",
+        help="Run historical backfill instead of starting the server",
+    )
+    parser.add_argument(
+        "--show", type=int,
+        help="Anchor episode number (required with --init)",
+    )
+    parser.add_argument(
+        "--aired", type=date.fromisoformat,
+        help="Air date of anchor episode as YYYY-MM-DD (required with --init)",
+    )
+    args = parser.parse_args()
+    if args.init and (args.show is None or args.aired is None):
+        parser.error("--init requires both --show and --aired")
+    return args
+
+
 def run() -> None:
+    args = _parse_args()
     settings = Settings()
 
     db = Database(settings.db_path)
     db.initialize()
     logger.info("Database initialized at %s", settings.db_path)
 
+    if args.init:
+        sc = SoundCloudClient()
+        asyncio.run(
+            run_backfill(
+                db=db,
+                soundcloud=sc,
+                soundcloud_user=settings.soundcloud_user,
+                show_day=settings.show_day,
+                show_hour=settings.show_hour,
+                anchor_episode=args.show,
+                anchor_aired=args.aired,
+            )
+        )
+        asyncio.run(sc.close())
+        logger.info("Backfill complete")
+        return
+
     sc = SoundCloudClient()
     poller = Poller(
         db=db,