Add LanceDB compaction to reclaim disk space

[rendro]

Summary

• Add sediment compact CLI command that runs LanceDB's three-phase optimization (compact fragments, prune old versions, rebuild indices) and reports disk space reclaimed
• Add automatic background compaction every 50 store/forget operations with conservative settings (single thread, prune versions >1 day old)
• Add Database::optimize_tables() core method with configurable CompactConfig

Fixes unbounded storage growth caused by LanceDB's append-only architecture — every store, delete, consolidation merge, and FTS rebuild creates new data files without reclaiming space.

Usage

# Manual compaction (reclaim all space)
sediment compact

# Also delete unverified files (<7 days old)
sediment compact --force

# Skip version pruning
sediment compact --no-prune

Test plan

• cargo fmt — no formatting issues
• cargo build --release — compiles cleanly
• cargo test — all 80 tests pass (29 ignored as before, require model download)
• sediment compact --help — shows correct usage
• Manual: du -sh ~/.sediment/data/ before and after sediment compact on a production database