feat(database): store gameplay replays in compact form

- Add compact storage package: codec, records, hydrate, replay blobs, cache.
- Running games write game_turns and snake_states keyed by integer ids.
- Encode moves as 1-byte codes and bodies as head plus step codes.
- Store food and hazards only when they changed since the previous turn.
- Strip reasoning fields that duplicate stored values and restore on read.
- Pack finished games into one zstd blob per game in game_replays.
- Merge late turns into packed blobs during the stale-game sweep.
- Extract SQLite and PostgreSQL schemas into dedicated modules.
- Add read/import game bundle and list_game_ids to the backend template.
- Add legacy readers and converter for the old turns/snake_turns layout.
- Add migrate_gameplay_to_compact.py with in-place or cross-DB migration.
- Verify every migrated game and keep legacy rows on failure.
- Support --since, --minimum-quality and guarded --drop-legacy.
- Add just recipe migrate-gameplay-postgres.
- Rewrite merge script to copy game by game via backend bundles.
- Move benchmark state loading into benchmark_states for both backends.
- Update PostgreSQL cleanup script to the new replay tables.
- Drop sqlite-zstd build stages from the Dockerfile.
- Remove old migration scripts, normalized_turn and their tests.
- Add tests for compact storage, schema, legacy migration and PostgreSQL.
- Document the compact layout and migration workflow in the README.

server/database/compact/quality.py

@@ -0,0 +1,23 @@
+"""Rate a game's replay quality from its records."""
+
+from server.database.game_quality import GameQuality, GameQualityInput, rate_game_quality
+
+from .codec import MOVES
+from .records import GameBundle
+
+def rate_bundle(bundle:GameBundle) -> GameQuality:
+  turns = bundle.turns
+  numbers = [record.turn for record in turns]
+  moves = [record.move for record in turns]
+  return rate_game_quality(GameQualityInput(
+    status=bundle.game.get("status") or "running",
+    final_turn=int(bundle.game.get("final_turn") or 0),
+    turn_rows=len(turns),
+    min_turn=min(numbers) if numbers else None,
+    max_turn=max(numbers) if numbers else None,
+    valid_moves=sum(1 for move in moves if move in MOVES),
+    thinking_rows=sum(1 for record in turns if record.thinking not in (None, {})),
+    distinct_moves=len({move for move in moves if move is not None}),
+    snake_turn_rows=len(bundle.states),
+    winner_name=bundle.game.get("winner_name"),
+  ))