Run SQLx migrations at startup

Add an initial services table migration and enable the sqlx
"migrate" feature. Introduce run_migrations in db.rs, run
migrations on startup, and pass the PgPool into the router.

migrations/20260116092854_initial_schema.sql

@@ -0,0 +1,27 @@
+-- Initial schema migration for services table
+-- This migration is idempotent and safe to run on existing databases
+
+-- Create services table if it doesn't exist
+CREATE TABLE IF NOT EXISTS services (
+    id SERIAL PRIMARY KEY,
+    token VARCHAR(128) NOT NULL UNIQUE,
+    "user" BIGINT NOT NULL,
+    status VARCHAR(12) NOT NULL,
+    created_time TIMESTAMPTZ NOT NULL,
+    cache VARCHAR(12) NOT NULL,
+    username VARCHAR(64) NOT NULL
+);
+
+-- Create unique index on token if it doesn't exist
+-- Note: The UNIQUE constraint already creates an index, but we ensure it exists
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1 FROM pg_indexes
+        WHERE tablename = 'services'
+        AND indexname = 'services_token_key'
+    ) THEN
+        CREATE UNIQUE INDEX services_token_key ON services(token);
+    END IF;
+END
+$$;