[Phase 4.1] Add TiKV client and define distributed schema

[zhexuany]

Summary

Add TiKV client integration and define the key-value schema for distributed coordination. This is the foundation for all distributed features.

Parent Epic

• [Epic] Distributed Roboflow with Alibaba Cloud (OSS + ACK) #9 Distributed Roboflow with TiKV Coordination

Dependencies

• None (foundation issue)
• Enables: [Phase 1.1] Add core dependencies for storage abstraction (#10) #29, [Phase 1.2] Define Storage trait and LocalStorage implementation (#11) #30, chore: update .gitignore #31, [Phase 2.1] Implement OSS/S3 backend using object_store #32, [Phase 5] Frame-level checkpoint with TiKV and multipart resume #19

TiKV Schema Design

Key Patterns

Key	Value	Purpose
/jobs/{file_hash}	JobRecord	Job definition and status
/locks/{resource}	LockRecord	Distributed locks with TTL
/state/{file_hash}	CheckpointState	Frame-level checkpoint
/heartbeat/{pod_id}	HeartbeatRecord	Worker liveness
/system/scanner_lock	LockRecord	Scanner leadership

JobRecord Schema

struct JobRecord {
    id: String,                    // UUID
    source_key: String,            // S3/OSS object key
    source_bucket: String,
    source_size: u64,
    status: JobStatus,             // Pending | Processing | Completed | Failed | Dead
    owner: Option<String>,         // Pod ID when Processing
    attempts: u32,
    max_attempts: u32,
    created_at: DateTime<Utc>,
    updated_at: DateTime<Utc>,
    error: Option<String>,
    output_prefix: String,
    config_hash: String,
}

Tasks

4.1.1 Add Dependencies

• Add tikv-client under feature flag distributed
• Add feature to Cargo.toml

4.1.2 Create Module Structure

• src/distributed/mod.rs
• src/distributed/tikv/ submodule

4.1.3 Define Schema Types

• JobRecord, JobStatus, LockRecord, HeartbeatRecord
• Serde serialization support

4.1.4 Implement Key Builders

• Consistent key prefix: /roboflow/v1/
• Static methods for each key type

4.1.5 Implement TiKV Client Wrapper

• Connection pooling
• Basic CRUD: get, put, delete, scan
• Batch operations: batch_get, batch_put

4.1.6 Implement Transactional Operations

• CAS (Compare-And-Swap) for atomic updates
• Multi-key transactions

4.1.7 Add Configuration

• TikvConfig with pd_endpoints, timeouts
• Environment variable support: TIKV_PD_ENDPOINTS

4.1.8 Error Handling

• TikvError enum with retry logic
• Map to RoboflowError

Acceptance Criteria

• Feature flag distributed compiles
• Schema types defined with serde
• TikvClient connects to cluster
• CRUD and batch operations work
• Transactional CAS works
• Configuration via env vars
• Unit tests pass

Files to Create

• src/distributed/mod.rs
• src/distributed/tikv/mod.rs
• src/distributed/tikv/client.rs
• src/distributed/tikv/schema.rs
• src/distributed/tikv/error.rs
• src/distributed/tikv/config.rs

[zhexuany]

Implemented in PR #52. Ready for merge (requires admin/auto merge).