feat(server): scan library on startup by default + README first-run walkthrough

Make a fresh install usable out of the box and document the first-run flow
for the public-facing repo.

- Default scan_on_startup to true (Default() + config.example.yaml, which is
  the live config baked into the image). Previously false, so a fresh stack
  came up with an empty library and no hint to scan. Scans are incremental
  (mtime skip), so the per-restart cost is just a directory walk. Re-point
  the env-override test to exercise the override against the new default.
- README: add a "First run" walkthrough (register -> scan -> integrations ->
  install Android app -> invite users), each grounded in a real route.
- Add docs/screenshots/ with six captures, referenced via width-constrained
  <img> wrapped in a link (shrink inline + click to open full size).
  API token and invite token were cropped/redacted out of the captures
  before commit so no live credential lands in the public history.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/screenshots/README.md

@@ -0,0 +1,19 @@
+# README screenshots
+
+Drop the files below into this directory with these **exact names** — the
+root `README.md` already references them, so no Markdown editing is needed.
+Export as PNG. Capture while logged in as admin and *after* the first
+library scan so nothing looks empty; keep the same browser width and theme
+across all shots for a consistent look.
+
+| Filename | Route | What to capture |
+|---|---|---|
+| `home.png` | `/` | The home page with a populated library (system playlists / recently added) |
+| `register.png` | `/register` | The account-creation form on a fresh instance |
+| `library-scan.png` | `/admin` | The library scan running in stages (grab it mid-scan so stage tallies show) |
+| `integrations.png` | `/admin/integrations` | The ListenBrainz + Lidarr integration cards |
+| `android-download.png` | `/settings` | The "Install the Android app" download card |
+| `invite-users.png` | `/admin/users` | The invite-token generation / users list |
+
+Tip: to control display width in the README, you can swap the Markdown
+`![alt](path)` for an HTML tag, e.g. `<img src="docs/screenshots/home.png" width="800" alt="...">`.