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/querier.go

@@ -80,6 +80,11 @@ type Querier interface {
 	// MEDIA ITEM FORMATS QUERIES
 	// Create media item format
 	CreateMediaItemFormat(ctx context.Context, arg CreateMediaItemFormatParams) (MediaItemFormats, error)
+	// 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.
+	CreateMediaItemIfNew(ctx context.Context, arg CreateMediaItemIfNewParams) (MediaItems, error)
 	// Media Notes queries
 	CreateMediaNote(ctx context.Context, arg CreateMediaNoteParams) (MediaNotes, error)
 	CreateMediaNoteFull(ctx context.Context, arg CreateMediaNoteFullParams) (MediaNotes, error)
@@ -250,6 +255,7 @@ type Querier interface {
 	// ============================================
 	GetMediaItemByFilePathForSync(ctx context.Context, filePath string) (MediaItems, error)
 	GetMediaItemByKoboContentId(ctx context.Context, koboContentID pgtype.Text) (MediaItems, error)
+	GetMediaItemByLibraryAndPath(ctx context.Context, arg GetMediaItemByLibraryAndPathParams) (MediaItems, error)
 	// Get media item by OPF identifier
 	GetMediaItemByOPFIdentifier(ctx context.Context, opfIdentifier pgtype.Text) (MediaItems, error)
 	// Get media item by OPF UUID