Make storage paths configurable instead of hardcoded ./tmp #6

Problem#

Storage paths are hardcoded to ./tmp/blobs and ./tmp/manifests throughout the codebase. This appears in:

• src/health.rs:82-83 - Health check paths
• src/health.rs:114 - Storage accessibility check
• src/health.rs:119 - Writable check
• src/gc.rs:80-81 - GC manifest scanning
• src/gc.rs:158 - GC blob scanning
• src/gc.rs:233 - GC blob deletion paths
• src/storage.rs (likely) - Actual blob/manifest storage

The args.rs CLI parser may already have a --storage or --data-dir flag, but the hardcoded paths bypass it.

Proposed Solution#

• Add a --data-dir CLI argument (or use the existing one if present)
• Pass the configured path through App state
• Replace all hardcoded ./tmp/ references with the configured path
• Default to ./data or ./tmp for backwards compatibility