fix(files): review round 2, the server — locks the GC and folders read under, the mount check, the old mount name

- GC: the expiry warning marks each file conditionally on the expiry it saw, one commit each (a file
  deleted meanwhile no longer fails the run; one re-timed keeps its fresh expiry unwarned); eviction
  counts the disk again when a candidate was freed meanwhile, so it never evicts for space already free.
- Folders: a retention change locks the folder and its files, and a finishing file reads the retention
  under a share lock, so neither misses the other; a folder delete locks the files it takes home (a
  file moved out meanwhile stays put, one deleted meanwhile no longer fails the delete).
- Upload: the folder is checked before the probe and held only once the job names it.
- Dedup enqueue locks the cached asset, so a release meanwhile yields a fresh one instead of a 500.
- Mount check reads the kernel's mount table instead of comparing devices (the vfs storage driver gave
  a mounted volume the device of /), and only inside a container; the API also logs a fresh, empty
  volume in the library's place.
- Compose files fall back to DOWNLOAD_HOST_PATH; the upgrade notes cover the from-source pull.
- Folder routes call owned_or_404 directly.

backend/app/main.py

@@ -28,6 +28,7 @@ log = logs.get_logger(__name__)
 from app import bodylimit, collation, logsink, state, sysconfig  # noqa: E402 — logging is configured first
 from app.config import settings  # noqa: E402 — logging is configured first
 from app.db import SessionLocal, engine  # noqa: E402 — logging is configured first
+from app.downloads import service as download_service  # noqa: E402 — logging is configured first
 from app.downloads import storage  # noqa: E402 — logging is configured first
 from app.plex import stream as plex_stream  # noqa: E402 — logging is configured first
 from app.plex import subtitles as plex_subtitles  # noqa: E402 — logging is configured first
@@ -84,6 +85,18 @@ async def lifespan(app: FastAPI):
             # With the traceback: this catch spans a connection, a query and a coercion at boot,
             # and `%s` of the exception does not say which of them raised.
             log.exception("could not read the stored log level; staying at %s", current_level())
+        if not problem:
+            try:
+                left = download_service.library_left_behind(db, settings.files_root)
+            except Exception:  # a database not reachable yet: the check is a hint, never a gate
+                log.exception("could not check the files library against its database")
+            else:
+                if left:
+                    log.critical(
+                        "The files library is empty but the database names files: the volume "
+                        "mounted at %s is not the one that holds them. See docs/self-hosting.md.",
+                        settings.files_root,
+                    )
         if not state.is_configured(db):
             token = state.rotate_setup_token(db)
             url = f"{settings.app_base}/setup?token={token}"