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/handlers/stats.go

@@ -3,6 +3,7 @@ package handlers
 import (
 	"bookhoard/internal/database"
 	"bookhoard/internal/services"
+	"errors"
 	"net/http"
 
 	"github.com/google/uuid"
@@ -76,6 +77,42 @@ func (h *StatsHandler) ThisWeek(c *echo.Context) error {
 	return c.JSON(http.StatusOK, week)
 }
 
+// LibraryUsage handles GET /api/stats/library-usage?library_id=<uuid|omit>
+// — the /stats Library tab: media_items INVENTORY aggregates (items,
+// authors, narrators, size, audio hours, pages, top genres/authors,
+// longest/largest items) for one library or the All-libraries scope.
+// Family lens: a library_id outside the requester's visible set is a 403,
+// never a silent pass (AGENTS.md standing rule; history: 431fec0).
+// Omitted library_id aggregates across all visible libraries. No time
+// dimension — that is the reading tab's business.
+func (h *StatsHandler) LibraryUsage(c *echo.Context) error {
+	user, ok := mustUser(c)
+	if !ok {
+		return echo.NewHTTPError(http.StatusUnauthorized, "invalid user")
+	}
+
+	var libraryID pgtype.UUID
+	if raw := c.QueryParam("library_id"); raw != "" {
+		parsed, err := uuid.Parse(raw)
+		if err != nil {
+			return echo.NewHTTPError(http.StatusBadRequest, "invalid library id")
+		}
+		libraryID = pgtype.UUID{Bytes: parsed, Valid: true}
+	}
+
+	stats, err := h.statsService.LibraryUsage(c.Request().Context(),
+		pgtype.UUID{Bytes: user.ID.Bytes, Valid: true},
+		libraryID,
+	)
+	if err != nil {
+		if errors.Is(err, services.ErrLibraryNotVisible) {
+			return echo.NewHTTPError(http.StatusForbidden, "library not in the user's visible set")
+		}
+		return echo.NewHTTPError(http.StatusInternalServerError, err.Error())
+	}
+	return c.JSON(http.StatusOK, stats)
+}
+
 // BookStats handles GET /api/stats/books/:id — the per-book card on the
 // book detail page (lifetime totals + recent sessions).
 func (h *StatsHandler) BookStats(c *echo.Context) error {