feat(stats): Library tab — per-library inventory stats + audio-aware Reading tab

The /stats page gains a second tab (Reading | Library) implementing the
Audiobookshelf-style library dashboard (HANDOFF item 9): media_items
INVENTORY aggregates — items, hours/pages, distinct authors, size,
distinct narrators, Top Genres / Top Authors bars, Longest / Largest
items — for one library or the All-libraries aggregate.

- GET /api/stats/library-usage?library_id=<uuid|omit>; family lens
  enforced: a library_id outside the requester's visible set is a 403,
  never a silent pass (AGENTS.md standing rule; 431fec0 history)
- six sqlc queries with the visibility pattern + healthy-item filter
  (archived_at IS NULL AND missing_scan_count = 0); hours_mode lets the
  client swap the Hours/Pages card and the Longest ranking per scope
- Reading tab unit audit (HANDOFF item 3, settled 2026-10-16): minutes
  are the merged currency everywhere honest; only the PAGES unit splits
  — Pages card goes mode-aware (Pages · text / Listening Time in an
  audio-only pool) and the charts row gains Listening per Day (hrs)
  when audio minutes exist in the range; summary daily buckets gain
  audio_minutes split by the item's library type
- tab state in the URL (?tab=library&library_id=…); selector fed by the
  existing /api/libraries/visibility endpoint; main.js?v=4
- integration tests: family-lens 403 pin, hidden-library exclusion
  (differential), seeded text+audio aggregate correctness, auth gate
- docs/developer/api/stats/get_library_usage.md

Known-remaining suite failures are environmental: TestConflictList_Empty
+ TestWebSocketPingPong collide with the concurrently-running app session
(live emulator pushes + scans on the shared dev DB), and
TestAdminCreateUser's timezone expectation drifted from live system
settings — all three fail on the clean tree too.

internal/database/querier.go

@@ -80,6 +80,9 @@ type Querier interface {
 	CreateMediaHighlight(ctx context.Context, arg CreateMediaHighlightParams) (MediaHighlights, error)
 	CreateMediaHighlightFull(ctx context.Context, arg CreateMediaHighlightFullParams) (MediaHighlights, error)
 	// Media Items queries
+	// narrators/narrators_search are NOT NULL DEFAULT '{}' (A2): callers MUST
+	// pass empty slices, never Go nil (an explicit NULL INSERT bypasses the
+	// column default and 23502s). The API create handler normalizes nil.
 	CreateMediaItem(ctx context.Context, arg CreateMediaItemParams) (MediaItems, error)
 	// MEDIA ITEM FORMATS QUERIES
 	// Create media item format
@@ -88,6 +91,8 @@ type Querier interface {
 	// (library_id, file_path) this inserts NOTHING and returns no row
 	// (pgx.ErrNoRows) — the handler then answers 200 with the existing row
 	// and a duplicate flag instead of pretending it created something.
+	// narrators NOT NULL: callers pass empty slices, never nil (see
+	// CreateMediaItem note).
 	CreateMediaItemIfNew(ctx context.Context, arg CreateMediaItemIfNewParams) (MediaItems, error)
 	// Media Notes queries
 	CreateMediaNote(ctx context.Context, arg CreateMediaNoteParams) (MediaNotes, error)
@@ -250,6 +255,24 @@ type Querier interface {
 	GetLibraryTypeByName(ctx context.Context, name string) (LibraryTypes, error)
 	// Library Types queries
 	GetLibraryTypes(ctx context.Context) ([]LibraryTypes, error)
+	GetLibraryUsageAuthors(ctx context.Context, arg GetLibraryUsageAuthorsParams) ([]GetLibraryUsageAuthorsRow, error)
+	// ============================================================================
+	// LIBRARY USAGE STATS (the /stats Library tab; the StatisticsService
+	// LibraryUsage aggregate). media_items INVENTORY aggregates — what is in
+	// the library, not what was read. Every query carries BOTH filters:
+	//   $1 = single-library scope (NULL = all visible libraries — "All")
+	//   $2 = the requester's visible-library id set (NULL = no filter; the
+	//        default-ALLOW family lens). A non-visible $1 is REJECTED at the
+	//        service/handler layer with 403 — the SQL filter is defense only.
+	//   + the healthy-item filter, matched to GetVisibleLibraryMediaCounts:
+	//     archived_at IS NULL AND missing_scan_count = 0 (dead rows lie).
+	// ============================================================================
+	GetLibraryUsageCards(ctx context.Context, arg GetLibraryUsageCardsParams) (GetLibraryUsageCardsRow, error)
+	GetLibraryUsageGenres(ctx context.Context, arg GetLibraryUsageGenresParams) ([]GetLibraryUsageGenresRow, error)
+	GetLibraryUsageLargestItems(ctx context.Context, arg GetLibraryUsageLargestItemsParams) ([]GetLibraryUsageLargestItemsRow, error)
+	// The Longest Items panel, one query both modes: $3 = true ranks by
+	// AUDIO duration (audio present in scope), false by page_count (text).
+	GetLibraryUsageLongestItems(ctx context.Context, arg GetLibraryUsageLongestItemsParams) ([]GetLibraryUsageLongestItemsRow, error)
 	GetLibraryVisibility(ctx context.Context, arg GetLibraryVisibilityParams) (LibraryVisibility, error)
 	// ============================================================================
 	// LIBRARY WITH TYPE INFO QUERIES