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>

scripts/generate-secrets.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 # Generates random values for the secrets docker-compose.yml requires
 # (LITELLM_MASTER_KEY, LITELLM_SALT_KEY, LITELLM_DB_PASSWORD, UI_PASSWORD,
-# PGVECTOR_DB_PASSWORD, LITELLM_PGVECTOR_API_KEY) and writes them into .env —
+# PGVECTOR_DB_PASSWORD, MEMORY_RETRIEVAL_API_KEY) and writes them into .env —
 # creating it from .env.example first if it doesn't exist.
 #
 # ponytail: only fills in blank values, never overwrites ones you've already
@@ -27,6 +27,6 @@ set_if_blank LITELLM_SALT_KEY "$(openssl rand -hex 32)"
 set_if_blank LITELLM_DB_PASSWORD "$(openssl rand -hex 32)"
 set_if_blank UI_PASSWORD "$(openssl rand -hex 16)"
 set_if_blank PGVECTOR_DB_PASSWORD "$(openssl rand -hex 32)"
-set_if_blank LITELLM_PGVECTOR_API_KEY "$(openssl rand -hex 32)"
+set_if_blank MEMORY_RETRIEVAL_API_KEY "$(openssl rand -hex 32)"
 
-echo "Done. Review .env, then set OPENWEBUI_LITELLM_KEY, LITELLM_PGVECTOR_EMBEDDING_KEY, and SEARXNG_LAN_IP per docs/proxy-key-onboarding.md and docs/memory-knowledgebase.md."
+echo "Done. Review .env, then set OPENWEBUI_LITELLM_KEY, MEMORY_RETRIEVAL_EMBEDDING_KEY, and SEARXNG_LAN_IP per docs/proxy-key-onboarding.md and docs/memory-knowledgebase.md."