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>

package.json

@@ -15,6 +15,7 @@
     "db:push": "drizzle-kit push",
     "db:pull": "drizzle-kit pull",
     "db:studio": "drizzle-kit studio",
+    "db:seed": "tsx src/db/seed.ts",
     "lint": "eslint",
     "format": "prettier --write . && eslint --fix",
     "check": "prettier --check .",