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/001_extensions_and_enums.up.sql

@@ -0,0 +1,7 @@
+-- Enable pgcrypto for gen_random_uuid()
+CREATE EXTENSION IF NOT EXISTS "pgcrypto";
+
+-- Custom enum types
+CREATE TYPE user_role     AS ENUM ('guest', 'host', 'admin');
+CREATE TYPE export_type   AS ENUM ('zip', 'html');
+CREATE TYPE export_status AS ENUM ('pending', 'running', 'done', 'failed');