fix(ts-sdk): make pg an optional, ranged peer dependency

mem0ai declared pg as a required peer pinned to exactly 8.11.3, so npm
auto-installed it for every user (including hosted MemoryClient users)
and failed with ERESOLVE in any project already on a newer pg.

The pin existed because pgvector.ts imported pg at module load, so
mem0ai/oss crashed without it. PGVector now loads pg through loadPeer on
initialize(), like the other optional vector stores, which lets pg and
@types/pg become optional ^8.11.x peers. @types/jest moves from peer to
dev dependencies.

Closes #3307

docs/components/vectordbs/dbs/pgvector.mdx

@@ -5,6 +5,12 @@ description: "Use pgvector as a vector store in Mem0 for PostgreSQL-based vector
 
 [pgvector](https://github.com/pgvector/pgvector) is an open-source vector similarity search extension for Postgres. After connecting to Postgres, run `CREATE EXTENSION IF NOT EXISTS vector;` to create the vector extension.
 
+The TypeScript SDK loads the `pg` driver only when you use this store, so install it alongside `mem0ai`:
+
+```bash
+npm install pg
+```
+
 ### Usage
 
 <CodeGroup>