feat(progress): track per-user resume position server-side

Add position_seconds (+progress_updated_at) to video_states so watch progress
survives across devices and can drive a feed filter. New POST
/api/videos/{id}/progress checkpoints the player position (clearing trivially
-early and near-finished positions). Feed serialize exposes position_seconds and
a show=in_progress filter lists started-but-unfinished videos. Un-marking
'watched' now keeps a stored position instead of deleting the row.

backend/app/models.py

@@ -250,6 +250,14 @@ class VideoState(Base):
     # new | watched | saved | hidden
     status: Mapped[str] = mapped_column(String(12), default="new", server_default="new")
     watched_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    # Resume position (seconds into the video) for the in-app player. Orthogonal to
+    # status: a partially-watched video keeps status "new" but a non-zero position, so
+    # it can render a progress bar and match the "in progress" feed filter. 0 = not
+    # started / finished (trivially-early and near-finished positions are not stored).
+    position_seconds: Mapped[int] = mapped_column(
+        Integer, default=0, server_default="0", nullable=False
+    )
+    progress_updated_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
     )