fix(roms): repair multi-file ROM downloads broken by deferred file stats

PR #3425 dropped `lazy="joined"` from `RomFile.rom` and removed the
`joinedload(RomFile.rom)` from the ROM loaders to speed up the gallery
query. That left the `RomFile.rom` backref unpopulated. Single-file
downloads only read `RomFile.full_path` (built from `file_path`/
`file_name`), so they kept working, but multi-file (game folder)
downloads call `file_name_for_download()` / `is_top_level`, which read
`self.rom.full_path`. With no eager-loaded backref, that triggered a
lazy load on a detached instance once the handler session closed,
raising `DetachedInstanceError` and returning a 500.

Rather than reverting the loader changes (and the gallery gains), wire
the `RomFile.rom` backref up in Python from the parent ROM we already
hold in memory, via `set_committed_value`. This is zero extra DB cost
and only runs on the detail/download paths (`with_details` and
`get_roms_by_fs_name`); the optimized `filter_roms` gallery query is
untouched.

https://claude.ai/code/session_01PSXKmejPRzdxLFMN6P2QQ4

backend/handler/database/roms_handler.py

@@ -30,6 +30,7 @@ from sqlalchemy.orm import (
     selectinload,
     undefer,
 )
+from sqlalchemy.orm.attributes import set_committed_value
 from sqlalchemy.sql.elements import ColumnElement
 from sqlalchemy.sql.selectable import Select
 
@@ -123,6 +124,26 @@ def _create_metadata_id_case(
     )
 
 
+def _link_rom_files_to_parent(result: "Rom | Iterable[Rom] | None") -> None:
+    """Populate the `RomFile.rom` backref from the already-loaded parent ROM.
+
+    The detail/download loaders eager-load `Rom.files` but not the reverse
+    `RomFile.rom` relationship. `RomFile.full_path` / `is_top_level` /
+    `file_name_for_download` read `self.rom`, which would otherwise trigger a
+    lazy load that fails once the session closes (`DetachedInstanceError`,
+    surfacing as a 500 on multi-file ROM downloads). We already hold the parent
+    in memory, so wire the backref up directly instead of re-adding a per-file
+    `joinedload` (which would undo the gallery query gains from #3425).
+    """
+    if result is None:
+        return
+
+    roms = [result] if isinstance(result, Rom) else result
+    for rom in roms:
+        for file in rom.files:
+            set_committed_value(file, "rom", rom)
+
+
 def with_details(func):
     @functools.wraps(func)
     def wrapper(*args, **kwargs):
@@ -158,7 +179,9 @@ def with_details(func):
             undefer(Rom.multi_file),
             undefer(Rom.top_level_file_count),
         )
-        return func(*args, **kwargs)
+        result = func(*args, **kwargs)
+        _link_rom_files_to_parent(result)
+        return result
 
     return wrapper
 
@@ -993,6 +1016,7 @@ class DBRomsHandler(DBBaseHandler):
             .all()
         )
 
+        _link_rom_files_to_parent(roms)
         return {rom.fs_name: rom for rom in roms}
 
     @begin_session

backend/tests/handler/test_db_handler.py

@@ -14,7 +14,7 @@ from handler.database import (
 )
 from models.assets import Save, Screenshot, State
 from models.platform import Platform
-from models.rom import Rom
+from models.rom import Rom, RomFile
 from models.user import Role, User
 
 
@@ -73,6 +73,40 @@ def test_roms(rom: Rom, platform: Platform):
     assert len(roms) == 1
 
 
+def test_multi_file_rom_backref_survives_session_close(rom: Rom, platform: Platform):
+    """Multi-file ROM downloads read `file.rom.full_path` after the handler
+    session closes. The detail loaders eager-load `Rom.files` but not the
+    reverse `RomFile.rom` relationship, so without the backref being populated
+    this raises `DetachedInstanceError` (a 500 on the download endpoint).
+    """
+    folder_path = f"{rom.fs_path}/{rom.fs_name}"
+    for file_name in ("disc1.bin", "disc2.bin"):
+        db_rom_handler.add_rom_file(
+            RomFile(
+                rom_id=rom.id,
+                file_name=file_name,
+                file_path=folder_path,
+                file_size_bytes=1,
+            )
+        )
+
+    fetched = db_rom_handler.get_rom(rom.id)
+    assert fetched is not None
+    assert len(fetched.files) == 2
+
+    # These all dereference `file.rom` on a now-detached instance.
+    for file in fetched.files:
+        assert file.rom.full_path == folder_path
+        assert file.is_top_level
+        assert file.file_name_for_download() == file.file_name
+
+    # `get_roms_by_ids` (used by the bulk zip download) must behave the same.
+    by_ids = db_rom_handler.get_roms_by_ids([rom.id])
+    assert len(by_ids) == 1
+    for file in by_ids[0].files:
+        assert file.rom.full_path == folder_path
+
+
 def test_filter_last_played(rom: Rom, platform: Platform, admin_user: User):
     second_rom = db_rom_handler.add_rom(
         Rom(