RocksDB storage backend should support relative paths

## Problem The RocksDB storage backend validation rejects relative paths with: ``` RocksDB path must be absolute, got: ./data/indexes ``` This forces users to hardcode machine-specific absolute paths in their config files, which: 1. **Breaks portability** — configs can't be shared across machines, checked into version control, or used in containers without modification 2. **Breaks simplicity** — a user who just wants to store indexes next to their config file has to figure out and type the full absolute path 3. **Contradicts user expectations** — most tools that accept file paths (databases, log files, etc.) resolve relative paths against the working directory or config file location ## Proposed Solution Resolve relative paths against either: - The working directory at startup (simplest), or - The directory containing the config file (more predictable) If there's a concern about ambiguity, a warning log message like `"Resolving relative RocksDB path './data/indexes' against CWD: /home/user/project"` would address it without blocking the user. ## Current Workaround Use a full absolute path: ```yaml storageBackend: backend_type: rocksdb path: /home/username/project/data/indexes ``` ## References - Validation code: `lib/src/indexes/config.rs:100-106` - Introduced in PR #153