feat(service): implement RocksDB as high volume backend

[aldy505]

objectstore-service now implements the tiered-storage high-volume contract (Backend + HighVolumeBackend) over an embedded RocksDB, alongside the BigTable and in-memory implementations. Rows mirror Bigtable's cell model as flat KV cells keyed by storage path plus a two-byte column suffix, with exact point gets only.

Every cell value is framed with an 8-byte deadline prefix, which drives a compaction filter that physically drops expired rows without decoding their payloads. A single write lock serializes all mutating read-modify-write sequences while reads use a snapshot, reproducing InMemoryBackend's atomicity so compare-and-write cannot interleave. Change-stream write/update/delete reporting matches BigTableBackend.

[codecov]

Codecov Report

❌ Patch coverage is 98.37878% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.73%. Comparing base (f2a0115) to head (ff006f9).

Files with missing lines	Patch %	Lines
objectstore-service/src/backend/rocksdb.rs	98.37%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #689      +/-   ##
==========================================
+ Coverage   91.35%   91.73%   +0.37%     
==========================================
  Files         117      118       +1     
  Lines       24209    25566    +1357     
==========================================
+ Hits        22116    23452    +1336     
- Misses       2093     2114      +21     

Components	Coverage Δ
Rust Backend	95.06% <98.37%> (+0.23%)	⬆️
Rust Client	81.70% <ø> (ø)
Python Client	93.81% <ø> (ø)

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