feat: add capacity planning docs, image upload system, and platform health monitoring

Documentation:
- Add comprehensive capacity planning guide (docs/architecture/capacity-planning.md)
- Add operations docs: platform-health, capacity-monitoring, image-storage
- Link pricing strategy to capacity planning documentation
- Update mkdocs.yml with new Operations section

Image Upload System:
- Add ImageService with WebP conversion and sharded directory structure
- Generate multiple size variants (original, 800px, 200px)
- Add storage stats endpoint for monitoring
- Add Pillow dependency for image processing

Platform Health Monitoring:
- Add /admin/platform-health page with real-time metrics
- Show CPU, memory, disk usage with progress bars
- Display capacity thresholds with status indicators
- Generate scaling recommendations automatically
- Determine infrastructure tier based on usage
- Add psutil dependency for system metrics

Admin UI:
- Add Capacity Monitor to Platform Health section in sidebar
- Create platform-health.html template with stats cards
- Create platform-health.js for Alpine.js state management

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/routes/admin_pages.py

@@ -1200,3 +1200,27 @@ async def admin_code_quality_violation_detail(
             "violation_id": violation_id,
         },
     )
+
+
+# ============================================================================
+# PLATFORM HEALTH & MONITORING ROUTES
+# ============================================================================
+
+
+@router.get("/platform-health", response_class=HTMLResponse, include_in_schema=False)
+async def admin_platform_health(
+    request: Request,
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render platform health monitoring page.
+    Shows system metrics, capacity thresholds, and scaling recommendations.
+    """
+    return templates.TemplateResponse(
+        "admin/platform-health.html",
+        {
+            "request": request,
+            "user": current_user,
+        },
+    )