Add SQLite content storage with queue-based operations

[dluc]

Summary

Implements SQLite-based content storage system with distributed locking and queue management.

Key Features

• Two-phase write pattern: Queue operation → Optimize queue → Process synchronously
• Distributed locking: Multi-VM support via LastAttemptTimestamp field
• Operations queue: Automatic cancellation of superseded operations
• Idempotent operations: Upsert (insert or replace) and Delete
• JSON storage: Tags as string arrays, metadata as key-value dictionaries
• Cuid2 IDs: Lowercase collision-resistant identifiers

Architecture

Database Schema

Content Table (km_content):

• Source of truth for all content
• String content with MIME type, byte size
• Ready flag for locking during operations
• Timestamps: ContentCreatedAt, RecordCreatedAt, RecordUpdatedAt
• Optional: Title, Description, Tags[], Metadata{}

Operations Table (km_operations):

• Queue management per ContentId
• Timestamp-ordered processing
• Multi-step support (single step in this implementation)
• Tracks: PlannedSteps, CompletedSteps, RemainingSteps
• LastAttemptTimestamp for distributed locking

Components

• Entity classes with JSON property handling
• EF Core DbContext with SQLite configuration
• Service layer implementing two-phase write pattern
• DTOs for clean API boundaries
• Custom exception types
• Cuid2-based ID generation

Test Coverage

74 tests, 100% passing:

• 43 configuration tests (existing)
• 31 storage tests (19 unit + 12 integration)

Implementation Details

• Zero build warnings (strict compliance)
• DateTimeOffset for all timestamps (culture-invariant ISO 8601)
• Hardcoded table names with km_ prefix to avoid collisions
• No recovery logic in user requests (deferred to future background worker)
• Comprehensive error handling and logging

Files Changed

New files (10):

• src/Core/Storage/ - Complete implementation
• tests/Core.Tests/Storage/ - Test suite

Modified files:

• src/Core/Core.csproj - Added Cuid.Net package
• Test visibility changes for xUnit compliance
• Package version management