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

tests/test_api.py

@@ -62,6 +62,7 @@ def test_playlist(client, db):
     resp = client.get("/playlist")
     assert resp.status_code == 200
     data = resp.json()
+    assert "episode_number" in data
     assert len(data["tracks"]) == 2
     assert data["tracks"][0]["position"] == 1
     assert data["tracks"][0]["title"] == "Song A"
@@ -84,14 +85,18 @@ def test_shows_list(client, db):
     _seed_show(db)
     resp = client.get("/shows")
     assert resp.status_code == 200
-    assert len(resp.json()) >= 1
+    data = resp.json()
+    assert len(data) >= 1
+    assert "episode_number" in data[0]
 
 
 def test_shows_detail(client, db):
     show = _seed_show(db)
     resp = client.get(f"/shows/{show.id}")
     assert resp.status_code == 200
-    assert len(resp.json()["tracks"]) == 2
+    data = resp.json()
+    assert "episode_number" in data
+    assert len(data["tracks"]) == 2
 
 
 def test_admin_refresh_requires_token(client):