feat(m5d): demand-driven deep backfill + per-user ETA

Per-user opt-in to full-history (deep) backfill so a new user's unique
channels no longer trigger a big shared-quota burst.

- migration 0007: Subscription.deep_requested (default false); seed admins'
  existing subscriptions to preserve today's global-backfill behaviour
- run_deep_backfill is now demand-driven: only channels at least one user has
  requested are deep-backfilled; recent backfill stays unconditional (cheap)
- estimate_deep_backfill ETA helper (quota-bound) surfaced in /api/sync/my-status
- POST /api/sync/deep-all to opt all my channels in; PATCH channels accepts
  deep_requested
- UI: per-channel Full history toggle, Backfill everything action, deep
  progress + ETA in Channels header and Settings - Sync

backend/alembic/versions/0007_deep_requested.py

@@ -0,0 +1,37 @@
+"""per-user opt-in to full-history (deep) backfill
+
+Revision ID: 0007_deep_requested
+Revises: 0006_app_state
+Create Date: 2026-06-11
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0007_deep_requested"
+down_revision: Union[str, None] = "0006_app_state"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "subscriptions",
+        sa.Column(
+            "deep_requested",
+            sa.Boolean(),
+            nullable=False,
+            server_default="false",
+        ),
+    )
+    # Preserve today's behaviour: admins already had every channel deep-backfilling
+    # globally, so opt their existing subscriptions in. Other users start recent-only.
+    op.execute(
+        "UPDATE subscriptions SET deep_requested = true "
+        "WHERE user_id IN (SELECT id FROM users WHERE role = 'admin')"
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("subscriptions", "deep_requested")