feat(db): hash_conflicts table and backfill/conflict queries

Content duplicates (same library + file_sha256 at different paths,
e.g. the same book imported twice under two names on a preexisting
database) cannot be auto-collapsed the way path duplicates were:
keeping both copies may be intentional. Surface them for an explicit
admin decision instead.

Schema:
- new hash_conflicts table keyed (library_id, file_sha256) with a
  status/resolution lifecycle: 'pending' until an admin resolves via
  'keep_all' or 'kept:<uuid>' (which copy was kept after merging)
- resolution is VARCHAR(50) - 'kept:<uuid>' is 41 chars; include a
  widening ALTER for databases created with the initial 30-char width
- resolution/resolved_by/resolved_at record who decided what and when

Queries:
- ListMediaItemsMissingHash: items imported before hashing existed
  (file_sha256 IS NULL), ordered oldest-first for the backfill pass
- FindHashConflictGroups: the content-duplicate group detection
  (GROUP BY library_id, file_sha256 HAVING COUNT(*) > 1)
- ListMediaItemsBySHA256AndLibrary: full membership of one group
- CreateHashConflict: upsert with DO NOTHING so already-tracked groups
  are untouched - critical behavior: a group an admin resolved as
  'keep both' is never re-flagged by later sweeps
- ListPendingHashConflicts: admin listing with library name and live
  item counts (items may have been deleted since flagging)
- GetHashConflict / ResolveHashConflict: lifecycle
- GetMediaItemUsageCounts: per-item progress/highlight/bookmark/note/
  collection counts so the admin can make an informed keep choice
- ReparentMediaItemChildren: sqlc binding for the existing
  reparent_media_item_children() migration function, used to merge a
  losing copy's child rows into the kept copy

internal/database/querier.go

@@ -53,6 +53,10 @@ type Querier interface {
 	// Create device shelf mapping
 	CreateDeviceShelfMapping(ctx context.Context, arg CreateDeviceShelfMappingParams) (DeviceShelfMappings, error)
 	CreateDictionaryEntry(ctx context.Context, arg CreateDictionaryEntryParams) (DictionaryCache, error)
+	// HASH CONFLICTS QUERIES
+	// Record a pending hash conflict (no-op if the group is already tracked, so
+	// resolved groups stay resolved and are never re-flagged)
+	CreateHashConflict(ctx context.Context, arg CreateHashConflictParams) error
 	// Libraries queries
 	CreateLibrary(ctx context.Context, arg CreateLibraryParams) (Libraries, error)
 	CreateMediaBookmark(ctx context.Context, arg CreateMediaBookmarkParams) (MediaBookmarks, error)
@@ -129,6 +133,8 @@ type Querier interface {
 	DeleteUnlinkedBook(ctx context.Context, id pgtype.UUID) error
 	DeleteUser(ctx context.Context, id pgtype.UUID) error
 	DeleteUserSystemCollection(ctx context.Context, arg DeleteUserSystemCollectionParams) error
+	// Find content-duplicate groups (same library + SHA-256, more than one row)
+	FindHashConflictGroups(ctx context.Context) ([]FindHashConflictGroupsRow, error)
 	GenerateKoboEntitlementId(ctx context.Context) (interface{}, error)
 	// ============================================
 	// ANNOTATION SERVE QUERIES
@@ -184,6 +190,7 @@ type Querier interface {
 	GetFailedSyncQueueItems(ctx context.Context, limit int32) ([]SyncQueue, error)
 	GetFirstAdmin(ctx context.Context) (pgtype.UUID, error)
 	GetFirstAdminExclude(ctx context.Context, id pgtype.UUID) (GetFirstAdminExcludeRow, error)
+	GetHashConflict(ctx context.Context, id pgtype.UUID) (HashConflicts, error)
 	GetKoboEntitlementByContentId(ctx context.Context, arg GetKoboEntitlementByContentIdParams) (GetKoboEntitlementByContentIdRow, error)
 	GetKoboEntitlementByEntitlementId(ctx context.Context, arg GetKoboEntitlementByEntitlementIdParams) (GetKoboEntitlementByEntitlementIdRow, error)
 	GetKoboEntitlementsForDevice(ctx context.Context, deviceID pgtype.UUID) ([]GetKoboEntitlementsForDeviceRow, error)
@@ -240,6 +247,8 @@ type Querier interface {
 	GetMediaItemFormatByType(ctx context.Context, arg GetMediaItemFormatByTypeParams) (MediaItemFormats, error)
 	// Get media item formats
 	GetMediaItemFormats(ctx context.Context, mediaItemID pgtype.UUID) ([]MediaItemFormats, error)
+	// Per-item user-data counts, used when choosing which duplicate copy to keep
+	GetMediaItemUsageCounts(ctx context.Context, mediaItemID pgtype.UUID) (GetMediaItemUsageCountsRow, error)
 	GetMediaNote(ctx context.Context, id pgtype.UUID) (MediaNotes, error)
 	// ============================================
 	// ANNOTATION SYNC QUERIES (notes)
@@ -323,7 +332,12 @@ type Querier interface {
 	ListLibraries(ctx context.Context) ([]ListLibrariesRow, error)
 	ListMediaItems(ctx context.Context, arg ListMediaItemsParams) ([]ListMediaItemsRow, error)
 	ListMediaItemsByLibrary(ctx context.Context, libraryID pgtype.UUID) ([]ListMediaItemsByLibraryRow, error)
+	// List all media items sharing a SHA-256 hash within a library (hash conflict group)
+	ListMediaItemsBySHA256AndLibrary(ctx context.Context, arg ListMediaItemsBySHA256AndLibraryParams) ([]MediaItems, error)
+	// List media items that have no stored SHA-256 (imported before hashing existed)
+	ListMediaItemsMissingHash(ctx context.Context) ([]MediaItems, error)
 	ListMediaItemsSorted(ctx context.Context, arg ListMediaItemsSortedParams) ([]ListMediaItemsSortedRow, error)
+	ListPendingHashConflicts(ctx context.Context) ([]ListPendingHashConflictsRow, error)
 	ListPendingSyncQueueItems(ctx context.Context, arg ListPendingSyncQueueItemsParams) ([]SyncQueue, error)
 	ListProcessingIssuesByLibrary(ctx context.Context, libraryID pgtype.UUID) ([]ListProcessingIssuesByLibraryRow, error)
 	ListSyncConflictsByMediaItem(ctx context.Context, arg ListSyncConflictsByMediaItemParams) ([]SyncConflicts, error)
@@ -341,7 +355,10 @@ type Querier interface {
 	// Remove book from collection
 	RemoveBookFromCollection(ctx context.Context, arg RemoveBookFromCollectionParams) error
 	RemoveBookFromKoboShelf(ctx context.Context, arg RemoveBookFromKoboShelfParams) error
+	// Re-parent all child rows of p_source onto p_target (defined in schema.sql)
+	ReparentMediaItemChildren(ctx context.Context, arg ReparentMediaItemChildrenParams) error
 	ResetSystemCollectionMetadata(ctx context.Context, arg ResetSystemCollectionMetadataParams) error
+	ResolveHashConflict(ctx context.Context, arg ResolveHashConflictParams) error
 	ResolveProcessingIssue(ctx context.Context, arg ResolveProcessingIssueParams) (ProcessingIssues, error)
 	ResolveSyncConflict(ctx context.Context, arg ResolveSyncConflictParams) (SyncConflicts, error)
 	// Resolve unlinked book