Benchmarks and metrics collection for launch blog post

## Summary

Once apex is functional end-to-end, collect concrete benchmarks and operational data for a single namespace to support a launch blog post. The post should tell the story of why apex exists, what problems it solves, and back it up with real numbers.

## Data to collect

### Storage
- Total celestia-node disk usage for the same height range (full node)
- Total apex disk usage for a single namespace over the same range
- Storage reduction ratio (e.g., "400GB → 2GB = 200x reduction")
- DB file size growth rate per day/week
- Breakdown: headers vs blob data vs indexes

### Performance
- Backfill throughput: heights/sec, blobs/sec during historical sync
- `blob.Get` latency: apex (SQLite lookup) vs celestia-node (namespace scan)
- `blob.GetAll` latency at various heights (sparse vs dense namespace)
- `blob.Subscribe` end-to-end latency: new block on celestia → blob delivered to consumer
- Time to full sync from a given start height

### Resource footprint
- Memory usage: idle, during backfill, during streaming
- CPU usage: idle, during backfill, during streaming
- Peak memory during heaviest operation
- Compare against celestia-node light/full node memory profile

### Simplicity
- Lines of code: apex total vs celestia-node
- Dependencies: count of direct deps in go.mod (core module, excluding submit/)
- Binary size
- Config file: apex YAML vs celestia-node setup ceremony (init, keys, trusted hash, etc.)
- Time from `git clone` to serving blobs (setup friction)

### Reliability
- Uptime over test period
- Sync gap recovery time
- Graceful restart time (shutdown → serving again)

## Blog post outline

1. **The problem** — celestia-node stores everything, rollups need almost nothing. Hundreds of GB for a few namespaces worth 10-20GB.
2. **What we tried / issues we hit** — reference the celestia-node issues discovered during research:
- 8-second blob retrieval times (celestia-node#4453)
- BadgerDB corruption on ungraceful shutdown (celestia-node#3881)
- Non-contiguous subscriptions (celestia-node#3578)
- No namespace-scoped storage despite 3+ year roadmap item (celestia-node#2033)
- Subscription buffer overflow with silent disconnect
- <10% resource utilization during sync (celestia-node#4108)
3. **The approach** — lightweight namespace indexer, SQLite, pluggable fetcher, drop-in API compatibility
4. **The numbers** — storage, performance, memory, simplicity benchmarks from above
5. **What's next** — Fiber support, gRPC, tx submission, multi-account

## When

After Phase 2 is complete (API layer working, can serve ev-node). Benchmarks should be run against mainnet data with a real namespace.

## Related

- #7 — Observability (metrics infrastructure needed to collect this data)
- #16 — Health endpoints (uptime, sync state data source)