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/queries/queries.sql

@@ -1712,6 +1712,66 @@ SELECT * FROM media_items WHERE file_sha256 = $1;
 -- name: GetMediaItemBySHA256AndLibrary :one
 SELECT * FROM media_items WHERE file_sha256 = $1 AND library_id = $2;
 
+-- List all media items sharing a SHA-256 hash within a library (hash conflict group)
+-- name: ListMediaItemsBySHA256AndLibrary :many
+SELECT * FROM media_items WHERE file_sha256 = $1 AND library_id = $2 ORDER BY file_path;
+
+-- List media items that have no stored SHA-256 (imported before hashing existed)
+-- name: ListMediaItemsMissingHash :many
+SELECT * FROM media_items WHERE file_sha256 IS NULL ORDER BY created_at;
+
+-- Find content-duplicate groups (same library + SHA-256, more than one row)
+-- name: FindHashConflictGroups :many
+SELECT library_id, file_sha256, COUNT(*) AS dup_count
+FROM media_items
+WHERE file_sha256 IS NOT NULL
+GROUP BY library_id, file_sha256
+HAVING COUNT(*) > 1;
+
+-- Per-item user-data counts, used when choosing which duplicate copy to keep
+-- name: GetMediaItemUsageCounts :one
+SELECT
+    (SELECT COUNT(*) FROM reading_progress  rp WHERE rp.media_item_id = $1) AS progress_count,
+    (SELECT COUNT(*) FROM media_highlights   mh WHERE mh.media_item_id = $1) AS highlights_count,
+    (SELECT COUNT(*) FROM media_bookmarks    mb WHERE mb.media_item_id = $1) AS bookmarks_count,
+    (SELECT COUNT(*) FROM media_notes        mn WHERE mn.media_item_id = $1) AS notes_count,
+    (SELECT COUNT(*) FROM collection_items   ci WHERE ci.media_item_id = $1) AS collections_count;
+
+-- 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)
+-- name: CreateHashConflict :exec
+INSERT INTO hash_conflicts (library_id, file_sha256)
+VALUES ($1, $2)
+ON CONFLICT (library_id, file_sha256) DO NOTHING;
+
+-- name: ListPendingHashConflicts :many
+SELECT hc.id, hc.library_id, hc.file_sha256, hc.created_at,
+       l.name AS library_name,
+       COUNT(mi.id) AS item_count
+FROM hash_conflicts hc
+JOIN libraries l ON l.id = hc.library_id
+LEFT JOIN media_items mi ON mi.library_id = hc.library_id AND mi.file_sha256 = hc.file_sha256
+WHERE hc.status = 'pending'
+GROUP BY hc.id, hc.library_id, hc.file_sha256, hc.created_at, l.name
+ORDER BY hc.created_at;
+
+-- name: GetHashConflict :one
+SELECT * FROM hash_conflicts WHERE id = $1;
+
+-- name: ResolveHashConflict :exec
+UPDATE hash_conflicts
+SET status = 'resolved',
+    resolution = $2,
+    resolved_by = $3,
+    resolved_at = NOW()
+WHERE id = $1;
+
+-- Re-parent all child rows of p_source onto p_target (defined in schema.sql)
+-- name: ReparentMediaItemChildren :exec
+SELECT reparent_media_item_children($1::uuid, $2::uuid);
+
 -- Get media item by OPF identifier
 -- name: GetMediaItemByOPFIdentifier :one
 SELECT * FROM media_items WHERE opf_identifier = $1;