Add new RocksDBPartitionedIndexBackend

### Task Description

**What needs to be done:**
New class implementing PartitionedIndexBackend (get(partitionPath, recordKey) / update(...)) backed. It contains column-family manager: create-if-absent, "register in default CF only after full load" (completeness invariant), drop CF, list registered partitions. No bootstrap/TTL yet — just point get/update against an already-open RocksDB instance. Add a PartitionedIndexBackendFactory (or extend IndexBackendFactory) to construct it behind the new config flag.

**Why this task is needed:**

### Task Type

Code improvement/refactoring

### Related Issues

**Parent feature issue:** (if applicable )
**Related issues:**
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