Add scheduled automatic garbage collection #8

Problem#

Garbage collection only runs when manually triggered via POST /admin/gc. There is no way to schedule periodic automatic GC, meaning unreferenced blobs accumulate until an admin remembers to trigger cleanup.

Proposed Solution#

• Add a --gc-interval CLI argument (e.g., --gc-interval 24h)
• Spawn a background tokio task that runs GC on the configured interval
• Use the existing gc::run_gc() function with configurable dry-run and grace period
• Log GC results and expose last-GC metrics (timestamp, blobs freed, bytes freed)
• Add grain_gc_last_run_timestamp and grain_gc_bytes_freed_total Prometheus gauges/counters
• Keep the manual POST /admin/gc endpoint for on-demand runs

Files to modify#

• src/main.rs - Spawn background GC task
• src/args.rs - Add --gc-interval and --gc-grace-period args
• src/metrics.rs - Add GC metrics