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/db.rs

@@ -0,0 +1,19 @@
+use anyhow::{Context, Result};
+use sqlx::postgres::PgPoolOptions;
+use sqlx::PgPool;
+
+pub async fn create_pool(database_url: &str) -> Result<PgPool> {
+    let pool = PgPoolOptions::new()
+        .max_connections(10)
+        .connect(database_url)
+        .await
+        .context("failed to connect to database")?;
+
+    sqlx::migrate!()
+        .run(&pool)
+        .await
+        .context("failed to run database migrations")?;
+
+    tracing::info!("database connected and migrations applied");
+    Ok(pool)
+}