`EphemeralKBService` manages temporary knowledge bases with TTL-based expiration. Extracted from `KBService` in 0.18.
Methods
| Method | Description | |--------|-------------| | `create_ephemeral_kb()` | Create an ephemeral KB with TTL, register in config and DB | | `gc_ephemeral_kbs()` | Garbage-collect expired ephemeral KBs, remove files and config |