feat: implement dependency-ordered task execution

Add dependency tracking and validation for tasks to enforce execution order
(schema changes → backend → UI) as specified in Section 1.3 of ralph-features-spec.md.

Changes:
- Add depends_on field to Task model (Vec<Uuid>) for explicit dependencies
- Create database migration for depends_on column with GIN index
- Add dependency_analysis.rs module with:
  - can_start_task() for checking if all dependencies are complete
  - Auto-detection of dependency patterns from file paths
  - Detection of schema/types/backend/UI categories
  - Warnings for potential dependency violations
- Add DependencyOrderingConfig to daemon config with:
  - enabled: Enable/disable dependency checking
  - auto_detect: Auto-detect dependencies from file patterns
  - warn_on_violation: Warn on detected violations
- Integrate dependency checks into task manager scheduling
- Add depends_on to DaemonCommand::SpawnTask protocol

The daemon performs dependency validation as a sanity check but defers to
the server for authoritative scheduling decisions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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