feat(security): library visibility enforced at the source; per-library series; annotation ownership; admin user creation

The family-lens hardening: library_visibility was only enforced where
file bytes flowed (download/reader/OPDS) — metadata surfaces leaked
hidden libraries to any authenticated account, and registration was
open by default with no admin-side user creation.

- Series are grouped PER LIBRARY now (same name in two libraries =
  two entries: manga adaptations / LN splits no longer merge);
  /api/series aggregates only the requester's VISIBLE libraries and
  /api/series/books REQUIRES a visible library_id (breaking contract;
  web + app clients updated in the same change).
- /api/media-items, item detail, and the reader content endpoints
  (chapters/panels/outline/THUMBNAILS — page images leaked unguarded
  next to a gated GetPage) all enforce the visible set; resolveLibrary
  no longer honors a hand-typed library_id outside the visible set.
- Annotation/note/bookmark mutations (update/delete/tombstone, sync
  LWW paths incl. conflict resolution) carry ownership predicates:
  another account's bare row id is a 404, not a silent delete.
- Admin user creation: POST /api/auth/users (AdminMiddleware) + a
  Create User card in the admin Users UI — accounts stay creatable
  with self-service registration OFF, which is now the schema default
  (fresh installs; the first-admin bootstrap exception is unchanged).
- Tests: series contract re-pinned (per-library, visibility 403s),
  new ownership + admin-create integration suites, OPDS all=1 test
  now passes an explicit page size (shared-DB growth made the fresh
  books fall off page 1), device-cap helper switched to the admin
  endpoint.

internal/router/helpers.go

@@ -3,6 +3,7 @@ package router
 import (
 	"context"
 	"log"
+	"net/http"
 	"net/url"
 	"os"
 	"time"
@@ -118,6 +119,15 @@ func resolveLibrary(c *echo.Context, cfg *Config, userUUID string) LibraryResolu
 		log.Printf("GetUserVisibleLibraries failed: %v", err)
 		libraries = []database.GetUserVisibleLibrariesRow{}
 	}
+	visibleSet := make(map[string]bool, len(libraries))
+	for _, lib := range libraries {
+		if !lib.ID.Valid {
+			continue
+		}
+		if id, err := uuid.FromBytes(lib.ID.Bytes[0:16]); err == nil {
+			visibleSet[id.String()] = true
+		}
+	}
 
 	counts, countErr := cfg.Queries.GetVisibleLibraryMediaCounts(c.Request().Context(), uuidToPGType(userU))
 	if countErr != nil {
@@ -189,8 +199,47 @@ func resolveLibrary(c *echo.Context, cfg *Config, userUUID string) LibraryResolu
 		return res
 	}
 
+	// A requested library outside the requester's visible set is not
+	// honored — fall back to the first visible library instead of leaking
+	// hidden-library contents through a hand-typed query param or cookie.
+	if !visibleSet[libraryID] {
+		res.LibraryID = res.FirstID
+		if res.LibraryID != "" {
+			parsed, _ := uuid.Parse(res.LibraryID)
+			res.LibUUID = pgtype.UUID{Bytes: parsed, Valid: true}
+		}
+		return res
+	}
+
 	res.LibraryID = libraryID
 	parsed, _ := uuid.Parse(libraryID)
 	res.LibUUID = pgtype.UUID{Bytes: parsed, Valid: true}
 	return res
 }
+
+// visibleLibraryIDsFor returns the requester's visible-library id set (for
+// membership checks) plus the ids themselves (for SQL ANY() filters).
+func visibleLibraryIDsFor(c *echo.Context, cfg *Config) (map[string]bool, []pgtype.UUID, error) {
+	userUUIDStr, ok := c.Get("user_id").(string)
+	if !ok {
+		return nil, nil, echo.NewHTTPError(http.StatusUnauthorized, "invalid user")
+	}
+	userU, err := uuid.Parse(userUUIDStr)
+	if err != nil {
+		return nil, nil, echo.NewHTTPError(http.StatusUnauthorized, "invalid user")
+	}
+	libraries, err := cfg.Queries.GetUserVisibleLibraries(c.Request().Context(), uuidToPGType(userU))
+	if err != nil {
+		return nil, nil, err
+	}
+	set := make(map[string]bool, len(libraries))
+	ids := make([]pgtype.UUID, 0, len(libraries))
+	for _, lib := range libraries {
+		if !lib.ID.Valid {
+			continue
+		}
+		set[lib.ID.String()] = true
+		ids = append(ids, lib.ID)
+	}
+	return set, ids, nil
+}