fix: strip markdown code fences from LLM JSON response

Gemini wraps JSON output in ```json fences which broke the parser.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/ai_analysis.py

@@ -259,6 +259,9 @@ async def _call_openai(system: str, user_prompt: str) -> str:
 
 def _parse_clips(content: str, video_duration: float) -> list[dict]:
     """Parse LLM response into clip list, handling imperfect JSON."""
+    # Strip markdown code fences (e.g. ```json ... ```)
+    content = re.sub(r"```(?:json)?\s*", "", content).strip()
+
     # Try to extract JSON from response
     json_match = re.search(r"\{[\s\S]*\}", content)
     if not json_match: