feat(server): POST /api/media-items answers honestly on a duplicate file_path

The create endpoint shared the scanner's upsert (ON CONFLICT
(library_id, file_path) DO UPDATE SET updated_at = NOW() RETURNING *) —
so a POST naming an already-registered file returned 201 CREATED with
the OLD row while silently discarding everything the caller submitted.
Nothing depended on that lie: the app only GETs/PUTs media items, the
web only searches/purges/deletes, KOReader never calls it — the
endpoint's create path had zero production callers.

Split the API path from the scanner's idempotent upsert, which is kept
verbatim in CreateMediaItem (re-scan dedup remains its safety net):

- CreateMediaItemIfNew: same insert with ON CONFLICT DO NOTHING — the
  loser of a create race writes nothing and returns no row.
- GetMediaItemByLibraryAndPath: fetch the existing row for the
  duplicate response.
- Handler branch: on pgx.ErrNoRows → 200 OK with
  {"duplicate": true, "message": ..., "item": <existing row>} instead
  of a 201 pretending it created something. Real creates still answer
  201 with the item as before.

Bulk-import shape (the reason for 200+flag over 409): a re-run after a
mid-import failure is harmless and countable — each response tells the
caller exactly which of the two things happened.

Verified by the full integration suite: 217 groups pass, zero failures,
including a new pin of the duplicate contract (same file_path twice in
one library → 201 then 200/duplicate:true, second payload not
overwritten).

internal/database/queries/queries.sql

@@ -154,6 +154,19 @@ VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16, $
 ON CONFLICT (library_id, file_path) DO UPDATE SET updated_at = NOW()
 RETURNING *;
 
+-- name: CreateMediaItemIfNew :one
+-- API create path: race-safe create-or-detect. On a duplicate
+-- (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.
+INSERT INTO media_items (library_id, title, author, isbn, description, file_path, file_size, mime_type, cover_image_path, series, series_number, tags, tags_search, asin, date_published, publisher, contributors, contributors_search, language, edition, page_count, genre, copyright_year, goodreads_id, openlibrary_id, google_books_id, added_by_admin_id, created_at, imported_at, manga_type, reading_direction, series_count, volume, imprint, age_rating, web_url, story_arc, is_black_and_white, metadata_notes, community_rating, alternate_info, scan_information, summary, library_type_name)
+VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16, $17, $18, $19, $20, $21, $22, $23, $24, $25, $26, $27, $28, $29, $30, $31, $32, $33, $34, $35, $36, $37, $38, $39, $40, $41, $42, $43, $44)
+ON CONFLICT (library_id, file_path) DO NOTHING
+RETURNING *;
+
+-- name: GetMediaItemByLibraryAndPath :one
+SELECT * FROM media_items WHERE library_id = $1 AND file_path = $2;
+
 -- name: GetMediaItem :one
 SELECT * FROM media_items WHERE id = $1;