Fix SVG generation: read from file instead of return value

🐛 Critical fix:
- KerykeionChartSVG.makeSVG() saves to file, doesn't return content
- Create temp directory for SVG output
- Read generated SVG file content
- Return file content in HTTP response

✅ Now SVG generation works correctly!

References:
- https://github.com/g-battaglia/kerykeion
- https://www.kerykeion.net/pydocs/kerykeion.html

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

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

app.py

@@ -177,10 +177,22 @@ async def generate_chart_svg(req: BirthRequest):
         )
 
         # Generate SVG chart (Kerykeion 4.x)
-        chart = KerykeionChartSVG(subject)
-
-        # Get SVG content as string
-        svg_content = chart.makeSVG()
+        # makeSVG() saves to file, doesn't return content
+        import tempfile
+        from pathlib import Path
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            output_dir = Path(tmpdir)
+            chart = KerykeionChartSVG(subject, new_output_directory=str(output_dir))
+            chart.makeSVG()
+
+            # Find the generated SVG file
+            svg_files = list(output_dir.glob("*.svg"))
+            if not svg_files:
+                raise HTTPException(status_code=500, detail="No SVG file generated")
+
+            svg_file = svg_files[0]
+            svg_content = svg_file.read_text(encoding='utf-8')
 
         if not svg_content or len(svg_content) < 100:
             raise HTTPException(status_code=500, detail="SVG generation returned empty content")