feat(embeddings): per-chunk rows — schema, write path, version-aware backfill (#280 steps 2+3)

note_embeddings becomes one row per chunk: PK (note_id, chunk_index), plus
chunk_text (what this vector actually encodes) and chunker_version. Migration
0077 clears the table — embeddings are derived (0067 precedent) and the old
whole-document rows are indistinguishable from single-chunk notes, so the
startup backfill regenerates the corpus at the new shape. The backfill is now
version-aware: a future shape change is a CHUNKER_VERSION bump that re-embeds
exactly the stale notes, not another wipe.

upsert_note_embedding takes (title, body) and chunks internally — one path
for the write path, the recurrence spawn and the backfill. The recurrence
spawn's own embed call is deleted outright: create_note already embeds via
embed_note (#2056), so the spawn was a second copy of the rule. An emptied
record now CLEARS its stale vectors instead of leaving them findable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

alembic/versions/0077_chunked_embeddings.py

@@ -0,0 +1,54 @@
+"""Chunked embeddings: one note_embeddings row per chunk (#280)
+
+Revision ID: 0077
+Revises: 0076
+Create Date: 2026-08-09
+
+The embedding model reads at most 512 tokens and fastembed truncates the rest
+silently, so the old one-row-per-note shape permanently lost everything past
+~400 words of a record. A note now stores one row per chunk of
+`embeddings.chunk_document`: PK (note_id, chunk_index), plus the chunk's text
+(inspectability + future "matched section" surfacing) and the chunker version
+that produced it (so later shape changes re-embed by version comparison
+instead of repeating this wipe).
+
+Embeddings are DERIVED data (0067 precedent): rows are cleared here and the
+startup backfill regenerates the whole corpus at the new shape on next boot.
+The HNSW index is untouched — it indexes chunk rows exactly as it indexed
+note rows.
+"""
+from alembic import op
+
+
+revision = "0077"
+down_revision = "0076"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Derived data — the version-aware startup backfill re-embeds everything
+    # at the chunked shape. Old whole-document rows would be indistinguishable
+    # from properly-chunked single-chunk notes, so they cannot be carried over.
+    op.execute("DELETE FROM note_embeddings")
+
+    # Empty table, so NOT NULL columns need no defaults and the PK swap is
+    # instant.
+    op.execute("ALTER TABLE note_embeddings ADD COLUMN chunk_index integer NOT NULL")
+    op.execute("ALTER TABLE note_embeddings ADD COLUMN chunk_text text NOT NULL")
+    op.execute("ALTER TABLE note_embeddings ADD COLUMN chunker_version integer NOT NULL")
+    op.execute("ALTER TABLE note_embeddings DROP CONSTRAINT note_embeddings_pkey")
+    op.execute(
+        "ALTER TABLE note_embeddings ADD PRIMARY KEY (note_id, chunk_index)"
+    )
+
+
+def downgrade() -> None:
+    # Same reasoning in reverse: chunk rows make no sense to a whole-document
+    # reader, so clear and let the old backfill regenerate.
+    op.execute("DELETE FROM note_embeddings")
+    op.execute("ALTER TABLE note_embeddings DROP CONSTRAINT note_embeddings_pkey")
+    op.execute("ALTER TABLE note_embeddings DROP COLUMN chunk_index")
+    op.execute("ALTER TABLE note_embeddings DROP COLUMN chunk_text")
+    op.execute("ALTER TABLE note_embeddings DROP COLUMN chunker_version")
+    op.execute("ALTER TABLE note_embeddings ADD PRIMARY KEY (note_id)")