docs: adopt Coolify-managed Postgres over SQLite

docs/adr/0003-use-coolify-managed-postgres-over-sqlite.md

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+# Use Coolify-managed PostgreSQL over SQLite
+
+The `/deep-design` session chose SQLite on a persistent volume for its zero-ops
+simplicity at this scale. Deploying on **Coolify** changes the calculus: Coolify
+provides **managed PostgreSQL with automatic scheduled backups**, whereas a
+SQLite file inside an app volume has *no* managed backup and forces a
+single-instance, stop-then-start deploy strategy to avoid file-lock contention
+when Coolify briefly runs old and new containers during a rolling restart. We
+switch to **Coolify-managed PostgreSQL** (via Drizzle's `pg` driver).
+
+## Consequences
+
+- Backups are handled by Coolify's database backup feature — no bespoke tooling
+  (e.g. Litestream) needed.
+- Deploys no longer need a recreate/single-instance constraint; the app can
+  restart freely while Postgres persists as a separate service.
+- Adds a Postgres service to local dev (`docker-compose`) and a managed DB in
+  prod — one more moving part than a single file.
+- **Supersedes** the SQLite choice in DESIGN.md (T5, F10) and the original
+  `/deep-design` decision.
+- Unchanged: deploys must **not** auto-run from `main` (every merge would deploy);
+  use a tagged release or manual trigger.