fix: add setup service for db push/seed; update README

The production runner image has no node_modules, so prisma CLI and tsx
are unavailable. Add a Compose 'setup' profile service that uses the
builder stage (which has all dev tools) to run db push and db seed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -26,14 +26,19 @@ Edit `.env`:
 - Change `POSTGRES_PASSWORD` to a strong password
 
 ```sh
-# 2. Build and start the stack
-docker compose up --build -d
+# 2. Build the images
+docker compose build
 
-# 3. First-run only: apply the schema and create your user
-docker compose exec app npx prisma db push
-docker compose exec app npx prisma db seed
+# 3. Start the database
+docker compose up db -d
 
-# 4. Open the app
+# 4. First-run only: apply the schema and create your user
+docker compose --profile setup run --rm setup
+
+# 5. Start the app
+docker compose up app -d
+
+# 6. Open the app
 open http://localhost:3000
 ```
 
@@ -65,8 +70,7 @@ docker compose down
 docker compose down -v
 
 # Re-seed after a wipe
-docker compose exec app npx prisma db push
-docker compose exec app npx prisma db seed
+docker compose --profile setup run --rm setup
 
 # Check health
 curl http://localhost:3000/api/health