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>

vendor/litellm-pgvector/prisma/schema.prisma

@@ -1,40 +0,0 @@
-// This is your Prisma schema file,
-// learn more about it in the docs: https://pris.ly/d/prisma-schema
-
-generator client {
-  provider = "prisma-client-py"
-}
-
-datasource db {
-  provider = "postgresql"
-  url      = env("DATABASE_URL")
-}
-
-model VectorStore {
-  id           String   @id @default(cuid())
-  name         String
-  file_counts  Json?
-  status       String   @default("completed")
-  usage_bytes  Int?     @default(0)
-  created_at   DateTime @default(now())
-  expires_after Json?
-  expires_at   DateTime?
-  last_active_at DateTime?
-  metadata     Json?
-  embeddings   Embedding[]
-  
-  @@map("vector_stores")
-}
-
-model Embedding {
-  id              String      @id @default(cuid())
-  vector_store_id String
-  content         String
-  embedding       Unsupported("vector(1536)")
-  metadata        Json?
-  created_at      DateTime    @default(now())
-  
-  vector_store    VectorStore @relation(fields: [vector_store_id], references: [id], onDelete: Cascade)
-  
-  @@map("embeddings")
-}