feat(storage): Add CQL high-volume backend

[lcian]

Adds a scylla-based CqlBackend implementing HighVolumeBackend for Cassandra and Scylla. It is selectable as type: cql either standalone or as the high-volume tier. Includes a pre-provisioned schema, optional authentication and verified TLS, a Cassandra devservice using the official image, CI coverage, and an example tiered configuration.

This draft shares the implementation for team review and possible sandbox benchmarking. Each object or upload marker occupies its own partition. Mutations use revision-guarded lightweight transactions, reads use LOCAL_SERIAL, and commits use LOCAL_QUORUM; clients are restricted to one configured datacenter. Logical expiration is backed by native per-cell TTL, so renewal rewrites the complete row. Unsupported deadlines at or beyond the documented 2038 ceiling are rejected.

The main benchmarking questions are throughput and tail latency with serial reads and conditional writes, contention on frequently updated keys, and the cost of renewing payloads near the 1 MiB tiering threshold. Replicated-cluster failure handling and authenticated TLS connections still need deployment-level validation before production use.

The full workspace suite passed (776 passed, 16 ignored). The 13-test CQL suite passed against Cassandra 5.0.9 and, unchanged, against a temporary Scylla 2026.1.14 instance. Both database runs used single-node clusters without authentication or TLS. The manual Scylla setup is not included in the commit; existing tiered-storage tests remain unchanged.

[codecov]

Codecov Report

❌ Patch coverage is 88.91626% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.38%. Comparing base (0abc6bf) to head (2f29312).

Files with missing lines	Patch %	Lines
objectstore-service/src/backend/cql.rs	89.85%	36 Missing ⚠️
objectstore-server/src/config.rs	87.50%	6 Missing ⚠️
objectstore-service/src/backend/mod.rs	0.00%	2 Missing ⚠️
objectstore-test/src/server.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #676    +/-   ##
========================================
  Coverage   91.38%   91.38%            
========================================
  Files         117      118     +1     
  Lines       24056    24459   +403     
========================================
+ Hits        21983    22353   +370     
- Misses       2073     2106    +33     

Components	Coverage Δ
Rust Backend	94.78% <89.13%> (-0.11%)	⬇️
Rust Client	81.70% <ø> (ø)
Python Client	93.81% <ø> (ø)

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