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.up.sql

@@ -0,0 +1,12 @@
+-- Seed default config values from .env defaults.
+-- ON CONFLICT DO NOTHING preserves any existing admin overrides.
+INSERT INTO config (key, value) VALUES
+    ('max_image_size_mb',       '20'),
+    ('max_video_size_mb',       '500'),
+    ('upload_rate_per_hour',    '10'),
+    ('feed_rate_per_min',       '60'),
+    ('export_rate_per_day',     '3'),
+    ('quota_tolerance',         '0.75'),
+    ('estimated_guest_count',   '100'),
+    ('compression_concurrency', '2')
+ON CONFLICT (key) DO NOTHING;