feat: ephemeral in-memory named vector store

[kordless]

Add a temporary session type that lives only in memory, never touches sled, and auto-expires 2 hours after the last ingest completes. Primary use case: crawl verification — ingest a batch, run queries against it from multiple angles, dump the full store, then it disappears.

Proposed API:

• `POST /temp/:name/ingest` — same body as regular ingest, stored only in RAM
• `GET /temp/:name/search?q=...` — same search params as regular session
• `GET /temp/:name/dump` — return all chunks (text + embeddings)
• `DELETE /temp/:name` — explicit teardown (auto-expire fires at 2h regardless)
• `GET /temp` — list active temp stores (name, chunk count, expires_at)

Implementation notes:

• `DashMap<String, TempStore>` in `AppState` with a TTL field set at completion of first ingest
• Background task sweeps expired entries every 10 minutes
• No sled writes — all vectors in process heap
• Expiry clock starts when the last ingest request to that name completes