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>

config.example.yaml

@@ -26,8 +26,13 @@ library:
   # Filesystem roots to scan for music. Each path is walked recursively.
   # Env: MINSTREL_LIBRARY_SCAN_PATHS (colon-separated, PATH-style)
   scan_paths: []
-  # Kick off a scan on server startup. Env: MINSTREL_LIBRARY_SCAN_ON_STARTUP
-  scan_on_startup: false
+  # Kick off a scan on server startup. On by default so a fresh install
+  # populates its library on first boot with no manual step. Scans are
+  # incremental (unchanged files are skipped by mtime), so the cost on
+  # every later restart is just a directory walk, not a full re-index.
+  # Set false if you'd rather scan only on a schedule / via the admin UI.
+  # Env: MINSTREL_LIBRARY_SCAN_ON_STARTUP
+  scan_on_startup: true
 
 # Per-instance branding. Operators can rename their instance ("Family
 # Jukebox", "Office Music", etc.) and override the OG share-preview