feat: implement v0.4.0 - Reports, Charts, Comparison, Dark Mode, E2E Testing

Backend (@backend-dev):
- Add ReportService with PDF/CSV generation (reportlab, pandas)
- Implement Report API endpoints (POST, GET, DELETE, download)
- Add ReportRepository and schemas
- Configure storage with auto-cleanup (30 days)
- Rate limiting: 10 downloads/minute
- Professional PDF templates with charts support

Frontend (@frontend-dev):
- Integrate Recharts for data visualization
- Add CostBreakdown, TimeSeries, ComparisonBar charts
- Implement scenario comparison page with multi-select
- Add dark/light mode toggle with ThemeProvider
- Create Reports page with generation form and list
- Add new UI components: checkbox, dialog, tabs, label, skeleton
- Implement useComparison and useReports hooks

QA (@qa-engineer):
- Setup Playwright E2E testing framework
- Create 7 test spec files with 94 test cases
- Add visual regression testing with baselines
- Configure multi-browser testing (Chrome, Firefox, WebKit)
- Add mobile responsive tests
- Create test fixtures and helpers
- Setup GitHub Actions CI workflow

Documentation (@spec-architect):
- Create detailed kanban-v0.4.0.md with 27 tasks
- Update progress.md with v0.4.0 tracking
- Create v0.4.0 planning prompt

Features:
✅ PDF/CSV Report Generation
✅ Interactive Charts (Pie, Area, Bar)
✅ Scenario Comparison (2-4 scenarios)
✅ Dark/Light Mode Toggle
✅ E2E Test Suite (94 tests)

Dependencies added:
- Backend: reportlab, pandas, slowapi
- Frontend: recharts, date-fns, @radix-ui/react-checkbox/dialog/tabs
- Testing: @playwright/test

27 tasks completed, 100% v0.4.0 implementation

src/repositories/report.py

@@ -0,0 +1,54 @@
+"""Report repository with specific methods."""
+
+from typing import Optional, List
+from uuid import UUID
+from datetime import datetime
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy import select, update, desc
+
+from src.models.report import Report
+from src.repositories.base import BaseRepository
+
+
+class ReportRepository(BaseRepository[Report]):
+    """Repository for Report model with specific methods."""
+
+    def __init__(self):
+        super().__init__(Report)
+
+    async def get_by_scenario(
+        self, db: AsyncSession, scenario_id: UUID, skip: int = 0, limit: int = 100
+    ) -> List[Report]:
+        """Get reports for a specific scenario."""
+        query = (
+            select(Report)
+            .where(Report.scenario_id == scenario_id)
+            .order_by(desc(Report.created_at))
+            .offset(skip)
+            .limit(limit)
+        )
+        result = await db.execute(query)
+        return result.scalars().all()
+
+    async def count_by_scenario(self, db: AsyncSession, scenario_id: UUID) -> int:
+        """Count reports for a specific scenario."""
+        query = select(Report).where(Report.scenario_id == scenario_id)
+        result = await db.execute(query)
+        return len(result.scalars().all())
+
+    async def update_file_size(
+        self, db: AsyncSession, report_id: UUID, file_size_bytes: int
+    ) -> Optional[Report]:
+        """Update report file size."""
+        result = await db.execute(
+            update(Report)
+            .where(Report.id == report_id)
+            .values(file_size_bytes=file_size_bytes)
+            .returning(Report)
+        )
+        await db.commit()
+        return result.scalar_one_or_none()
+
+
+# Singleton instance
+report_repository = ReportRepository()