feat(backend): Add Celery worker infrastructure with task stubs

- Add Celery app configuration with Redis broker/backend
- Add task modules: agent, workflow, cost, git, sync
- Add task stubs for:
  - Agent execution (spawn, heartbeat, terminate)
  - Workflow orchestration (start sprint, checkpoint, code review)
  - Cost tracking (record usage, calculate, generate report)
  - Git operations (clone, commit, push, sync)
  - External sync (import issues, export updates)
- Add task tests directory structure
- Configure for production-ready Celery setup

Implements #18

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

backend/app/tasks/__init__.py

@@ -0,0 +1,23 @@
+# app/tasks/__init__.py
+"""
+Celery background tasks for Syndarix.
+
+This package contains all Celery tasks organized by domain:
+
+Modules:
+    agent: Agent execution tasks (run_agent_step, spawn_agent, terminate_agent)
+    git: Git operation tasks (clone, commit, branch, push, PR)
+    sync: Issue synchronization tasks (incremental/full sync, webhooks)
+    workflow: Workflow state management tasks
+    cost: Cost tracking and budget monitoring tasks
+"""
+
+from app.tasks import agent, cost, git, sync, workflow
+
+__all__ = [
+    "agent",
+    "cost",
+    "git",
+    "sync",
+    "workflow",
+]