feat: migrate chunks with start_char/end_char columns

surfsense_backend/alembic/versions/166_add_chunk_char_spans.py

@@ -0,0 +1,31 @@
+"""add chunks.start_char/end_char for citation offsets
+
+Char offsets into the document's source_markdown (half-open span) let citations
+resolve the exact passage a chunk came from. Nullable because historical rows
+have no span; they populate on the next connector sync or user edit/reindex.
+
+No backfill: a bulk UPDATE of every chunk on a large HNSW-indexed table rewrites
+every secondary index per row (see migration 165 for the same reasoning).
+
+Revision ID: 166
+Revises: 165
+"""
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "166"
+down_revision: str | None = "165"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.execute("ALTER TABLE chunks ADD COLUMN IF NOT EXISTS start_char INTEGER;")
+    op.execute("ALTER TABLE chunks ADD COLUMN IF NOT EXISTS end_char INTEGER;")
+
+
+def downgrade() -> None:
+    op.execute("ALTER TABLE chunks DROP COLUMN IF EXISTS end_char;")
+    op.execute("ALTER TABLE chunks DROP COLUMN IF EXISTS start_char;")