fix(files): a file is added when it is done, not when it was asked for

A download that failed for days and then went through sat days down the library, under its
request date, and read as missing. A job now records when it became done (`ready_at`), stamped
where the expiry already is (`folders.stamp_done`, `finalize_if_running`) and on an insert that is
already done; a check constraint holds every path to done to it. The library sorts and dates its
"Added" by it; the queue keeps the request order.

The migration dates a file from before by its "ready to watch" notice where the inbox still holds
one, else by its request date, where it already sat.

backend/app/models.py

@@ -1,4 +1,4 @@
-from datetime import date, datetime
+from datetime import date, datetime, timezone
 
 from sqlalchemy import (
     BigInteger,
@@ -949,6 +949,13 @@ class FileFolder(Base, TimestampMixin):
     retention_days: Mapped[int | None] = mapped_column(Integer)
 
 
+def _ready_on_insert(context) -> datetime | None:
+    """A job inserted already done (an upload kept as it came) was ready the moment it was made; one
+    that becomes done later is stamped then (`folders.stamp_done`, `service.finalize_if_running`)."""
+    done = context.get_current_parameters().get("status") == "done"
+    return datetime.now(timezone.utc) if done else None
+
+
 class DownloadJob(Base, TimestampMixin, UpdatedAtMixin):
     """A user's request to download one source — the per-user layer over `MediaAsset`.
 
@@ -967,7 +974,13 @@ class DownloadJob(Base, TimestampMixin, UpdatedAtMixin):
     point at the parent download the clip was cut from; `edit_spec` is the (trim/crop) recipe."""
 
     __tablename__ = "download_jobs"
-    __table_args__ = (Index("ix_download_jobs_claim", "status", "queue_pos"),)
+    __table_args__ = (
+        Index("ix_download_jobs_claim", "status", "queue_pos"),
+        # Every way a job becomes done stamps when (`folders.stamp_done`, `finalize_if_running`).
+        CheckConstraint(
+            "status <> 'done' OR ready_at IS NOT NULL", name="ck_download_jobs_ready_at"
+        ),
+    )
 
     id: Mapped[int] = mapped_column(primary_key=True)
     user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), index=True)
@@ -1026,6 +1039,12 @@ class DownloadJob(Base, TimestampMixin, UpdatedAtMixin):
     kept: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
     expires_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), index=True)
     expiry_warned: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
+    # When the file became done — what the library's "Added" means: a download that failed for days
+    # and then went through is new to its user, so it sorts as new. `created_at` stays when it was
+    # asked for (the queue's order). Set again when a file is made anew (a retry, a re-download).
+    ready_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), default=_ready_on_insert
+    )
 
 
 class DownloadShare(Base, TimestampMixin):