feat(feed): accent-insensitive search (unaccent)

Feed text search used plain ILIKE, which is case- but not diacritic-insensitive, so 'tiesto'
missed the many titles spelled 'Tiësto' — a search that ingested ~45 results showed only ~12.
Enable the postgres unaccent extension (migration 0029) and wrap both sides of the title/
channel match in unaccent(), so 'tiesto' now matches 'Tiësto'. Applies to feed, count and
facets alike.

backend/alembic/versions/0029_unaccent_search.py

@@ -0,0 +1,31 @@
+"""accent-insensitive feed search (unaccent)
+
+Revision ID: 0029_unaccent_search
+Revises: 0028_live_search_provenance
+Create Date: 2026-06-29
+
+Enables the PostgreSQL `unaccent` contrib extension so the feed's text search can match
+regardless of diacritics — e.g. searching "tiesto" finds titles spelled "Tiësto". The feed
+query wraps both sides in unaccent(); this migration just makes the function available.
+`unaccent` ships with the official postgres image (postgresql-contrib). The DB role is a
+superuser in our deployments, so CREATE EXTENSION succeeds; IF NOT EXISTS keeps it idempotent.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0029_unaccent_search"
+down_revision: Union[str, None] = "0028_live_search_provenance"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute("CREATE EXTENSION IF NOT EXISTS unaccent")
+
+
+def downgrade() -> None:
+    # Intentionally left in place: other code/queries may rely on unaccent() and dropping a
+    # contrib extension is rarely what's wanted on a downgrade. (The feed query reverts with
+    # the code, not this migration.)
+    pass