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/alembic/versions/0010_watch_progress.py

@@ -0,0 +1,44 @@
+"""watch progress: resume position on video_states
+
+Revision ID: 0010_watch_progress
+Revises: 0009_quota_events
+Create Date: 2026-06-14
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0010_watch_progress"
+down_revision: Union[str, None] = "0009_quota_events"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "video_states",
+        sa.Column(
+            "position_seconds",
+            sa.Integer(),
+            nullable=False,
+            server_default="0",
+        ),
+    )
+    op.add_column(
+        "video_states",
+        sa.Column("progress_updated_at", sa.DateTime(timezone=True), nullable=True),
+    )
+    # Partial index to keep the "in progress" feed filter cheap (only started videos).
+    op.create_index(
+        "ix_video_states_in_progress",
+        "video_states",
+        ["user_id"],
+        postgresql_where=sa.text("position_seconds > 0"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_video_states_in_progress", table_name="video_states")
+    op.drop_column("video_states", "progress_updated_at")
+    op.drop_column("video_states", "position_seconds")