migration: clear note_embeddings for fastembed swap (768d → 384d)

JSONB column so no type change needed — just wipe and let the
startup backfill regenerate at the new dimension.

alembic/versions/0052_clear_note_embeddings_for_fastembed.py

@@ -0,0 +1,33 @@
+"""clear note_embeddings for fastembed swap
+
+Revision ID: 0052
+Revises: 0051
+Create Date: 2026-05-26
+
+Embeddings stored in `note_embeddings.embedding` (JSONB) were generated by
+Ollama's nomic-embed-text model (768-dim). The fastembed swap uses
+BAAI/bge-small-en-v1.5 (384-dim). Mixed-dim vectors would break the
+Python-side cosine similarity (length mismatch), so we wipe the table.
+
+The startup hook `services.embeddings.backfill_note_embeddings` regenerates
+everything at the new dimension on next boot. There's no column-type change
+because the column is JSONB — dimensionless on the storage side.
+
+Downgrade is the same operation: a fresh deployment of the prior code would
+backfill with the old model. No data is lost that the next boot won't restore.
+"""
+from alembic import op
+
+
+revision = "0052"
+down_revision = "0051"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute("DELETE FROM note_embeddings")
+
+
+def downgrade() -> None:
+    op.execute("DELETE FROM note_embeddings")