feat: M4 (part 1) — feed API, watch state, user preferences

- GET /api/feed: filter by tags (and/or), channel, duration, age, search; sort
  newest/oldest/views/duration/shuffle; default hides Shorts, live/upcoming and
  watched (was_live VODs stay visible); offset paging with has_more
- POST /api/videos/{id}/state for watched/saved/hidden/new
- User.preferences JSON + GET /api/me and PUT /api/me/preferences
- Migration 0004 (preferences column + video_states table)

backend/alembic/versions/0004_feed_state_prefs.py

@@ -0,0 +1,52 @@
+"""user preferences + per-user video watch state
+
+Revision ID: 0004_feed_state_prefs
+Revises: 0003_tags
+Create Date: 2026-06-11
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0004_feed_state_prefs"
+down_revision: Union[str, None] = "0003_tags"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("users", sa.Column("preferences", sa.JSON(), nullable=True))
+
+    op.create_table(
+        "video_states",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column(
+            "user_id",
+            sa.Integer(),
+            sa.ForeignKey("users.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column(
+            "video_id",
+            sa.String(length=16),
+            sa.ForeignKey("videos.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column("status", sa.String(length=12), nullable=False, server_default="new"),
+        sa.Column("watched_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column(
+            "updated_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.func.now(),
+            nullable=False,
+        ),
+        sa.UniqueConstraint("user_id", "video_id", name="uq_user_video"),
+    )
+    op.create_index("ix_video_states_user_id", "video_states", ["user_id"])
+    op.create_index("ix_video_states_video_id", "video_states", ["video_id"])
+
+
+def downgrade() -> None:
+    op.drop_table("video_states")
+    op.drop_column("users", "preferences")