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>

CLAUDE.md

@@ -44,12 +44,13 @@ src/
 ├── db/
 │   ├── schema.ts         # Drizzle schema definitions
 │   ├── index.ts          # Drizzle client instance
-│   └── migrations/       # generated SQL migrations — never hand-edit
+│   └── seed.ts            # sample data for local dev / tests
 ├── styles/
 │   ├── globals.scss      # global styles, resets
 │   └── _variables.scss   # shared SCSS variables/mixins (partial)
 public/                  # static assets
 drizzle.config.ts         # Drizzle Kit config (schema path, out dir, dialect)
+drizzle/                  # generated SQL migrations — never hand-edit
 ```
 
 ### Routing conventions
@@ -116,14 +117,17 @@ drizzle.config.ts         # Drizzle Kit config (schema path, out dir, dialect)
 - After changing `schema.ts`, generate a migration rather than editing the
   database by hand:
   ```bash
-  pnpm drizzle-kit generate   # create a migration from schema changes
-  pnpm drizzle-kit migrate    # apply pending migrations
+  npm run db:generate   # create a migration from schema changes
+  npm run db:migrate    # apply pending migrations
   ```
-- Never hand-edit files under `db/migrations/` — they're generated and
-  should be treated as a history, not touched retroactively.
-- [Connection details: env var name for `DATABASE_URL`, whether using a
-  connection pool like `postgres.js` or `node-postgres`, local dev DB setup
-  e.g. Docker Compose]
+- Never hand-edit files under `drizzle/` (the migrations output directory,
+  configured in `drizzle.config.ts`) — they're generated and should be
+  treated as a history, not touched retroactively.
+- Connection string comes from `DATABASE_URL` (see `.env.example`), read via
+  `drizzle-orm/node-postgres`. Local dev DB is a `docker-compose.yml`
+  Postgres container — see the README's "Local development database"
+  section. `.env.local` is picked up automatically by both the dev server
+  and `drizzle-kit`; no manual sourcing needed.
 
 ## Testing
 
@@ -134,7 +138,7 @@ drizzle.config.ts         # Drizzle Kit config (schema path, out dir, dialect)
 
 ## Things to avoid
 
-- Don't hand-edit `routeTree.gen.ts` or files under `db/migrations/`.
+- Don't hand-edit `routeTree.gen.ts` or files under `drizzle/`.
 - Don't run destructive Drizzle commands (`drizzle-kit push` against a
   populated DB, or anything that drops columns/tables) without confirming
   first — prefer `generate` + `migrate` so changes are reviewable.