refactor(catalog): the HTTP layer reads through queries and writes through the service

Channels, search and sync do no database work in their routes any more: reads go through
catalog/queries, writes through catalog/service, and a live search's ingest is catalog/search.
What a catalog route needs from Reader (a playlist's channels, a user's tag links, the feed
projection of results) it asks reader/queries and reader/service. The scheduler and the runs page
read channel titles and the work queue through catalog/queries. Catalog is fenced below its HTTP
layer: its owner filters go through userscope, it reads no other unit's tables, and its ingest
jobs commit only where they are named.

backend/app/catalog/service.py

@@ -0,0 +1,133 @@
+"""Catalog's writes a request makes: a user's subscription settings, what they explore or block, a
+subscription recorded or dropped, a channel's sync markers reset.
+
+Every function here flushes and leaves the commit to its caller (the handler). The YouTube side of a
+subscribe or an unsubscribe is the handler's (it needs the user's client and its refusals); what
+lands here is the local record of it. The ingest jobs (`sync/`, `catalog/search`) write the
+catalog's shared rows themselves.
+"""
+
+from datetime import datetime, timezone
+
+from sqlalchemy import delete, update
+from sqlalchemy.engine import CursorResult
+from sqlalchemy.orm import Session
+
+from app.catalog import queries
+from app.catalog.models import BlockedChannel, Channel, ExploredChannel, Subscription, Video
+from app.models import User
+from app.userscope import owned_by
+
+
+def remember_own_channel(user: User, channel_id: str) -> None:
+    """The user's own YouTube channel, so discovery never offers it to them."""
+    user.yt_channel_id = channel_id
+
+
+def update_subscription(
+    db: Session,
+    sub: Subscription,
+    *,
+    priority: int | None = None,
+    hidden: bool | None = None,
+    deep_requested: bool | None = None,
+) -> bool:
+    """Change the settings given (None leaves one as it is). Returns whether full history was just
+    turned ON — the caller then pulls the channel now rather than at the scheduler's next tick."""
+    turned_on = False
+    if priority is not None:
+        sub.priority = priority
+    if hidden is not None:
+        sub.hidden = hidden
+    if deep_requested is not None:
+        # Turning it off won't undo already-fetched videos; it stops further deep paging if nobody
+        # else still wants it.
+        turned_on = deep_requested and not sub.deep_requested
+        sub.deep_requested = deep_requested
+    db.flush()
+    return turned_on
+
+
+def set_deep_all(db: Session, user: User, on: bool) -> int:
+    """Every one of the user's channels in (or out of) full-history backfill; how many rows moved."""
+    result = db.execute(
+        update(Subscription).where(owned_by(Subscription.user_id, user)).values(deep_requested=on)
+    )
+    return result.rowcount if isinstance(result, CursorResult) else 0
+
+
+def explore(db: Session, user: User, channel: Channel) -> None:
+    """Record (or refresh the grace clock of) the user's exploration of a channel, and flag the
+    channel ephemeral unless someone follows it — the cleanup job's keep rule reads both."""
+    existing = queries.exploration(db, user, channel.id)
+    if existing is None:
+        db.add(ExploredChannel(user_id=user.id, channel_id=channel.id))
+    else:
+        existing.created_at = datetime.now(timezone.utc)
+    if not channel.from_explore and not queries.followed(db, channel.id):
+        channel.from_explore = True
+    db.flush()
+
+
+def reset_backfill(db: Session, channel: Channel) -> None:
+    """Fetch the channel again from scratch: its backfill markers cleared, and full history asked
+    for on EVERY subscription to it — the deep scheduler gates on an EXISTS over those, so the admin
+    who resets it need not subscribe themselves."""
+    channel.backfill_done = False
+    channel.backfill_cursor = None
+    channel.recent_synced_at = None
+    db.execute(
+        update(Subscription)
+        .where(Subscription.channel_id == channel.id)
+        .values(deep_requested=True)
+    )
+    db.flush()
+
+
+def record_subscription(db: Session, user: User, channel: Channel, yt_subscription_id: str) -> None:
+    """The subscription YouTube now holds, recorded here. Subscribing is the "keep" signal: an
+    explored channel becomes an organic one, so the cleanup job leaves it alone and its videos join
+    the shared Library like any subscribed channel's uploads."""
+    db.add(
+        Subscription(
+            user_id=user.id,
+            channel_id=channel.id,
+            yt_subscription_id=yt_subscription_id or None,
+            subscribed_at=datetime.now(timezone.utc),
+        )
+    )
+    if channel.from_explore:
+        channel.from_explore = False
+        db.execute(
+            update(Video)
+            .where(Video.channel_id == channel.id, Video.via_explore.is_(True))
+            .values(via_explore=False)
+        )
+    db.flush()
+
+
+def drop_subscription(db: Session, sub: Subscription) -> None:
+    db.delete(sub)
+    db.flush()
+
+
+def block(db: Session, user: User, channel_id: str) -> None:
+    """Block a channel for this user (idempotent), and end any exploration of it so the cleanup job
+    can reclaim it."""
+    if queries.is_blocked(db, user, channel_id):
+        return
+    db.add(BlockedChannel(user_id=user.id, channel_id=channel_id))
+    db.execute(
+        delete(ExploredChannel).where(
+            owned_by(ExploredChannel.user_id, user), ExploredChannel.channel_id == channel_id
+        )
+    )
+    db.flush()
+
+
+def unblock(db: Session, user: User, channel_id: str) -> None:
+    db.execute(
+        delete(BlockedChannel).where(
+            owned_by(BlockedChannel.user_id, user), BlockedChannel.channel_id == channel_id
+        )
+    )