feat(15-01): fix 13 markdown violations + wire md:lint to fast-checks CI step

- Fix MD040 (11 bare fences): add language tags (text/bash) across 7 files
- Fix MD031 (2 violations): add blank lines around fence in GETTING-STARTED.md
- Wire 'Markdown lint' step to fast-checks job (after Format check, before Typecheck)
- Reformat .markdownlint-cli2.jsonc per Prettier (trailing commas in JSONC)
- pnpm md:lint exits 0; pnpm format:check exits 0; gate can fail on bare fence (verified)

docs/GETTING-STARTED.md

@@ -49,9 +49,11 @@ Open `.env` and fill in the required values. See [docs/CONFIGURATION.md](CONFIGU
 
 - `DB_PASSWORD` and `DB_ROOT_PASSWORD` — pick any local passwords
 - `APP_PASSWORD_ENCRYPTION_KEY` — 64 hex characters; generate with:
+
   ```bash
   node -e "console.log(require('crypto').randomBytes(32).toString('hex'))"
   ```
+
 - `DEV_AUTH_BYPASS=true` — bypasses the live Authelia OIDC flow for local dev
 - `DB_HOST=localhost` — the dev Docker Compose exposes MariaDB on the host at `localhost:3306`