perf(db): drop 10 redundant single-column FK indexes (R12 S5)

Each column already leads a composite UNIQUE constraint whose btree covers
leftmost-prefix lookups on it (and the ON DELETE CASCADE reverse lookups),
so the extra single-column index=True was pure write-amplification. Drop
index=True on the 10 columns + alembic 0062 drops the existing indexes
(DROP INDEX IF EXISTS; downgrade recreates). Verified: migration up/down
round-trips on localdev (10 indexes drop, composites intact), siftlode
check green (alembic upgrade head incl. 0062 + pytest 193). The 10 do not
appear in an alembic-check model/DB diff → models and migration agree.

backend/alembic/versions/0062_drop_redundant_idx.py

@@ -0,0 +1,53 @@
+"""drop 10 redundant single-column FK indexes (R12 S5)
+
+Each of these columns already leads a composite UNIQUE constraint, whose btree covers leftmost-prefix
+lookups on that column (and the ON DELETE CASCADE reverse lookups too) — so the extra single-column
+`index=True` was pure write-amplification. Dropping them costs nothing on reads and saves an index
+write per insert/update. Fast (index-only drop, no table rewrite). Reversible (downgrade recreates).
+
+Redundant index -> the composite that already covers it:
+  ix_subscriptions_user_id            <- uq_user_channel (user_id, channel_id)
+  ix_video_states_user_id             <- uq_user_video (user_id, video_id)
+  ix_search_finds_user_id             <- uq_user_search_video (user_id, video_id)
+  ix_explored_channels_user_id        <- uq_user_explored_channel (user_id, channel_id)
+  ix_blocked_channels_user_id         <- uq_user_blocked_channel (user_id, channel_id)
+  ix_saved_views_user_id              <- uq_user_saved_view (user_id, name)
+  ix_playlist_items_playlist_id       <- uq_playlist_video (playlist_id, video_id)
+  ix_download_shares_job_id           <- uq_download_share (job_id, to_user_id)
+  ix_plex_states_user_id              <- uq_plex_user_item (user_id, item_id)
+  ix_plex_playlist_items_playlist_id  <- uq_plex_playlist_item (playlist_id, item_id)
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0062_drop_redundant_idx"
+down_revision: Union[str, None] = "0061_dl_job_error_code"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+# (index_name, table_name, column)
+_REDUNDANT: list[tuple[str, str, str]] = [
+    ("ix_subscriptions_user_id", "subscriptions", "user_id"),
+    ("ix_video_states_user_id", "video_states", "user_id"),
+    ("ix_search_finds_user_id", "search_finds", "user_id"),
+    ("ix_explored_channels_user_id", "explored_channels", "user_id"),
+    ("ix_blocked_channels_user_id", "blocked_channels", "user_id"),
+    ("ix_saved_views_user_id", "saved_views", "user_id"),
+    ("ix_playlist_items_playlist_id", "playlist_items", "playlist_id"),
+    ("ix_download_shares_job_id", "download_shares", "job_id"),
+    ("ix_plex_states_user_id", "plex_states", "user_id"),
+    ("ix_plex_playlist_items_playlist_id", "plex_playlist_items", "playlist_id"),
+]
+
+
+def upgrade() -> None:
+    for idx, table, _col in _REDUNDANT:
+        # IF EXISTS: a DB whose schema was stamped from a create_all (tests) won't have these once
+        # the models drop index=True, so a plain drop_index would error there.
+        op.execute(f"DROP INDEX IF EXISTS {idx}")
+
+
+def downgrade() -> None:
+    for idx, table, col in _REDUNDANT:
+        op.create_index(idx, table, [col])

backend/app/models.py

@@ -219,7 +219,7 @@ class Subscription(Base, TimestampMixin):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     user_id: Mapped[int] = mapped_column(
-        ForeignKey("users.id", ondelete="CASCADE"), index=True
+        ForeignKey("users.id", ondelete="CASCADE")
     )
     channel_id: Mapped[str] = mapped_column(
         ForeignKey("channels.id", ondelete="CASCADE"), index=True
@@ -388,7 +388,7 @@ class VideoState(Base, UpdatedAtMixin):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     user_id: Mapped[int] = mapped_column(
-        ForeignKey("users.id", ondelete="CASCADE"), index=True
+        ForeignKey("users.id", ondelete="CASCADE")
     )
     video_id: Mapped[str] = mapped_column(
         ForeignKey("videos.id", ondelete="CASCADE"), index=True
@@ -418,7 +418,7 @@ class SearchFind(Base, TimestampMixin):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     user_id: Mapped[int] = mapped_column(
-        ForeignKey("users.id", ondelete="CASCADE"), index=True
+        ForeignKey("users.id", ondelete="CASCADE")
     )
     video_id: Mapped[str] = mapped_column(
         ForeignKey("videos.id", ondelete="CASCADE"), index=True
@@ -443,7 +443,7 @@ class ExploredChannel(Base, TimestampMixin):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     user_id: Mapped[int] = mapped_column(
-        ForeignKey("users.id", ondelete="CASCADE"), index=True
+        ForeignKey("users.id", ondelete="CASCADE")
     )
     channel_id: Mapped[str] = mapped_column(
         ForeignKey("channels.id", ondelete="CASCADE"), index=True
@@ -462,7 +462,7 @@ class BlockedChannel(Base, TimestampMixin):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     user_id: Mapped[int] = mapped_column(
-        ForeignKey("users.id", ondelete="CASCADE"), index=True
+        ForeignKey("users.id", ondelete="CASCADE")
     )
     channel_id: Mapped[str] = mapped_column(
         ForeignKey("channels.id", ondelete="CASCADE"), index=True
@@ -619,7 +619,7 @@ class PlaylistItem(Base):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     playlist_id: Mapped[int] = mapped_column(
-        ForeignKey("playlists.id", ondelete="CASCADE"), index=True
+        ForeignKey("playlists.id", ondelete="CASCADE")
     )
     video_id: Mapped[str] = mapped_column(
         ForeignKey("videos.id", ondelete="CASCADE"), index=True
@@ -644,7 +644,7 @@ class SavedView(Base, TimestampMixin, UpdatedAtMixin):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     user_id: Mapped[int] = mapped_column(
-        ForeignKey("users.id", ondelete="CASCADE"), index=True
+        ForeignKey("users.id", ondelete="CASCADE")
     )
     name: Mapped[str] = mapped_column(String(80))
     filters: Mapped[dict] = mapped_column(JSON)
@@ -893,7 +893,7 @@ class DownloadShare(Base, TimestampMixin):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     job_id: Mapped[int] = mapped_column(
-        ForeignKey("download_jobs.id", ondelete="CASCADE"), index=True
+        ForeignKey("download_jobs.id", ondelete="CASCADE")
     )
     from_user_id: Mapped[int] = mapped_column(
         ForeignKey("users.id", ondelete="CASCADE"), index=True
@@ -1191,7 +1191,7 @@ class PlexState(Base, UpdatedAtMixin):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     user_id: Mapped[int] = mapped_column(
-        ForeignKey("users.id", ondelete="CASCADE"), index=True
+        ForeignKey("users.id", ondelete="CASCADE")
     )
     item_id: Mapped[int] = mapped_column(
         ForeignKey("plex_items.id", ondelete="CASCADE"), index=True
@@ -1260,7 +1260,7 @@ class PlexPlaylistItem(Base):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     playlist_id: Mapped[int] = mapped_column(
-        ForeignKey("plex_playlists.id", ondelete="CASCADE"), index=True
+        ForeignKey("plex_playlists.id", ondelete="CASCADE")
     )
     item_id: Mapped[int] = mapped_column(
         ForeignKey("plex_items.id", ondelete="CASCADE"), index=True