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>

internal/config/config.go

@@ -135,6 +135,11 @@ func Default() Config {
 			RadioSizeMax:        200,
 		},
 		Library: LibraryConfig{
+			// On by default so a fresh install populates its library on
+			// first boot without the operator hunting for a "scan now"
+			// button. Scans are incremental (unchanged files skipped by
+			// mtime), so every later restart is just a cheap directory walk.
+			ScanOnStartup:     true,
 			CoverArtFromMBCAA: true,
 		},
 	}

internal/config/config_test.go

@@ -97,7 +97,8 @@ func TestEnvOverrides(t *testing.T) {
 func TestLibraryEnvOverrides(t *testing.T) {
 	stubStreamSecret(t)
 	t.Setenv("MINSTREL_LIBRARY_SCAN_PATHS", "/music:/other::/third")
-	t.Setenv("MINSTREL_LIBRARY_SCAN_ON_STARTUP", "true")
+	// Default is true; set false so the override path is actually exercised.
+	t.Setenv("MINSTREL_LIBRARY_SCAN_ON_STARTUP", "false")
 
 	cfg, err := Load("")
 	if err != nil {
@@ -112,8 +113,8 @@ func TestLibraryEnvOverrides(t *testing.T) {
 			t.Errorf("scan_paths[%d] = %q, want %q", i, cfg.Library.ScanPaths[i], p)
 		}
 	}
-	if !cfg.Library.ScanOnStartup {
-		t.Error("scan_on_startup = false, want true")
+	if cfg.Library.ScanOnStartup {
+		t.Error("scan_on_startup = true, want false (env override of the true default)")
 	}
 }