feat(knowledgebase): replace litellm-pgvector connector with memory-retrieval

Per docs/research/langchain-pgvector-vs-litellm-pgvector.md (issue #25):
the vendored litellm-pgvector connector (793 lines, Prisma migrations, a
fragile git-context build) is replaced by a ~90-line FastAPI service
(services/memory-retrieval/) wrapping langchain_postgres.PGVector directly
against pgvector-db. Same gateway boundary — it still calls litellm for
embeddings, nothing talks to Postgres or the model directly except this
service.

- New services/memory-retrieval/ (main.py, Dockerfile, requirements.txt):
  POST /ingest, POST /query, GET /health.
- docker-compose.yml: litellm-pgvector service replaced by memory-retrieval;
  pgvector-db and embedding-server untouched.
- litellm-config.yaml: vector_store_registry block removed (no
  langchain_postgres provider exists to register against; callers query
  memory-retrieval directly instead of an in-band file_search tool call —
  that mechanism was never confirmed working per issue #24 anyway).
- scripts/ingest-memory.sh rewritten for the new /ingest endpoint (same
  per-line chunking, no dedup).
- .env vars renamed: LITELLM_PGVECTOR_API_KEY/LITELLM_PGVECTOR_EMBEDDING_KEY
  -> MEMORY_RETRIEVAL_API_KEY/MEMORY_RETRIEVAL_EMBEDDING_KEY.
- vendor/litellm-pgvector/ removed entirely.
- docs/memory-knowledgebase.md updated for the new setup/query flow.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -43,12 +43,12 @@ LITELLM_DB_PASSWORD=
 UI_USERNAME=admin
 UI_PASSWORD=
 
-# --- Knowledgebase (pgvector, see docs/memory-knowledgebase.md) ---
+# --- Knowledgebase (pgvector + memory-retrieval, see docs/memory-knowledgebase.md) ---
 # Required — random values, e.g. `openssl rand -hex 32`.
 PGVECTOR_DB_PASSWORD=
-# Auth key litellm-pgvector requires on its own API (its SERVER_API_KEY).
-LITELLM_PGVECTOR_API_KEY=
-# A virtual key litellm-pgvector uses to call back into litellm for
+# Auth key memory-retrieval requires on its own API (its SERVER_API_KEY).
+MEMORY_RETRIEVAL_API_KEY=
+# A virtual key memory-retrieval uses to call back into litellm for
 # embeddings — create it in the Admin UI like any other workload key
-# (see docs/proxy-key-onboarding.md), name it "litellm-pgvector".
-LITELLM_PGVECTOR_EMBEDDING_KEY=
+# (see docs/proxy-key-onboarding.md), name it "memory-retrieval".
+MEMORY_RETRIEVAL_EMBEDDING_KEY=