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/src/main.rs

@@ -1,6 +1,8 @@
 use anyhow::Result;
 use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
 
+mod db;
+
 #[tokio::main]
 async fn main() -> Result<()> {
     dotenvy::dotenv().ok();
@@ -12,10 +14,14 @@ async fn main() -> Result<()> {
         .with(tracing_subscriber::fmt::layer())
         .init();
 
+    let database_url =
+        std::env::var("DATABASE_URL").expect("DATABASE_URL must be set");
     let port: u16 = std::env::var("APP_PORT")
         .unwrap_or_else(|_| "3000".to_string())
         .parse()?;
 
+    let _pool = db::create_pool(&database_url).await?;
+
     let router = axum::Router::new()
         .route("/health", axum::routing::get(|| async { "ok" }));