feat: embeddings table + projector handlers (pure-Python cosine, T88)

chat/db/migrations/0012_embeddings.sql

@@ -0,0 +1,14 @@
+-- Embeddings stored as JSON arrays (pure-Python cosine at query time).
+-- Phase 4.5+ may swap to sqlite-vec when the host Python supports
+-- loadable extensions; the schema is intentionally simple to make that
+-- migration straightforward.
+CREATE TABLE embeddings (
+    memory_id INTEGER PRIMARY KEY,
+    vector_json TEXT NOT NULL,   -- JSON array of floats, length = dim
+    model TEXT NOT NULL,
+    dim INTEGER NOT NULL,
+    indexed_at TEXT NOT NULL DEFAULT (datetime('now')),
+    FOREIGN KEY (memory_id) REFERENCES memories(id)
+);
+
+CREATE INDEX embeddings_model_idx ON embeddings(model);