Add docker-compose for local Postgres and fix the setup docs

Adds docker-compose.yml (Postgres on host port 5433, to avoid clashing
with another project's container already using 5432 on this machine)
and .env.example so `docker compose up -d` + `cp .env.example .env.local`
gets a working local DB. Adds a db:seed script alongside the existing
db:* scripts.

Rewrites README's "Adding a Database" section, which still described
Better Auth's DB as optional/hypothetical even though this project
already requires Postgres for both its own data and Better Auth's
tables. Fills in CLAUDE.md's Data layer connection-details placeholder
and fixes two more stale pnpm references and a wrong migrations path
(src/db/migrations/ instead of the actual drizzle/) found nearby.

Also fixes a real bug the new docs surfaced: `npm run db:seed` didn't
load .env.local the way drizzle-kit does, so DATABASE_URL was silently
unset when run as documented. seed.ts now loads env the same way
drizzle.config.ts already does.

Verified the full documented flow end-to-end against a fresh container:
migrate, better-auth migrate, seed, dev server, real page render.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/db/seed.ts

@@ -88,6 +88,9 @@ export const seedGhostKitchen = async (db: NodePgDatabase<typeof schema>) => {
 }
 
 if (import.meta.url === `file://${process.argv[1]}`) {
+  const { config } = await import('dotenv')
+  config({ path: ['.env.local', '.env'] })
+
   const { drizzle } = await import('drizzle-orm/node-postgres')
   const db = drizzle(process.env.DATABASE_URL!, { schema })
   await seedGhostKitchen(db)