feat: add database schema and SQLx migrations

- 5 reversible migrations: extensions/enums, tables, indexes, views, config seed
- Tables: event, user, session, upload, hashtag, upload_hashtag, comment,
  comment_hashtag, like, export_job, config
- Views: v_feed (uploads with like/comment counts), v_hashtag_counts
- Indexes optimised for feed queries, session lookup, hashtag filtering
- Config table seeded with default rate limits and quotas
- db.rs module: PgPool creation with auto-migration on startup
- docker-compose.override.yml: expose db port 5432 for local dev
- Fix crate names: async_zip, tower_governor (underscore, not hyphen)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/migrations/005_seed_config.down.sql

@@ -0,0 +1,10 @@
+DELETE FROM config WHERE key IN (
+    'max_image_size_mb',
+    'max_video_size_mb',
+    'upload_rate_per_hour',
+    'feed_rate_per_min',
+    'export_rate_per_day',
+    'quota_tolerance',
+    'estimated_guest_count',
+    'compression_concurrency'
+);