feat: add change_seq to transcripts for ingestion support (#868)

* feat: add change_seq to transcripts for ingestion support

Add a monotonically increasing change_seq column to the transcript table,
backed by a PostgreSQL sequence and BEFORE INSERT OR UPDATE trigger. Every
mutation gets a new sequence value, letting external ingesters checkpoint
and never miss an update.

* chore: regenerate frontend API types

server/migrations/versions/623af934249a_add_change_seq_to_transcript.py

@@ -0,0 +1,74 @@
+"""add_change_seq_to_transcript
+
+Revision ID: 623af934249a
+Revises: 3aa20b96d963
+Create Date: 2026-02-19 18:53:12.315440
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "623af934249a"
+down_revision: Union[str, None] = "3aa20b96d963"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Sequence
+    op.execute("CREATE SEQUENCE IF NOT EXISTS transcript_change_seq;")
+
+    # Column (nullable first for backfill)
+    op.add_column("transcript", sa.Column("change_seq", sa.BigInteger(), nullable=True))
+
+    # Backfill existing rows with sequential values (ordered by created_at for determinism)
+    op.execute("""
+        UPDATE transcript SET change_seq = sub.seq FROM (
+            SELECT id, nextval('transcript_change_seq') AS seq
+            FROM transcript ORDER BY created_at ASC
+        ) sub WHERE transcript.id = sub.id;
+    """)
+
+    # Now make NOT NULL
+    op.alter_column("transcript", "change_seq", nullable=False)
+
+    # Default for any inserts between now and trigger creation
+    op.alter_column(
+        "transcript",
+        "change_seq",
+        server_default=sa.text("nextval('transcript_change_seq')"),
+    )
+
+    # Trigger function
+    op.execute("""
+        CREATE OR REPLACE FUNCTION set_transcript_change_seq()
+        RETURNS TRIGGER AS $$
+        BEGIN
+            NEW.change_seq := nextval('transcript_change_seq');
+            RETURN NEW;
+        END;
+        $$ LANGUAGE plpgsql;
+    """)
+
+    # Trigger (fires on every INSERT or UPDATE)
+    op.execute("""
+        CREATE TRIGGER trigger_transcript_change_seq
+            BEFORE INSERT OR UPDATE ON transcript
+            FOR EACH ROW
+            EXECUTE FUNCTION set_transcript_change_seq();
+    """)
+
+    # Index for efficient polling
+    op.create_index("idx_transcript_change_seq", "transcript", ["change_seq"])
+
+
+def downgrade() -> None:
+    op.execute("DROP TRIGGER IF EXISTS trigger_transcript_change_seq ON transcript;")
+    op.execute("DROP FUNCTION IF EXISTS set_transcript_change_seq();")
+    op.drop_index("idx_transcript_change_seq", table_name="transcript")
+    op.drop_column("transcript", "change_seq")
+    op.execute("DROP SEQUENCE IF EXISTS transcript_change_seq;")