fix: integrate placeholder resolver + JSON unwrapping (Issue #28)

- Backend: integrate get_placeholder_example_values in execute_prompt_with_data
- Backend: now provides BOTH raw data AND processed placeholders
- Backend: unwrap Markdown-wrapped JSON (```json ... ```)
- Fixes old-style prompts that expect name, weight_trend, caliper_summary

Resolves unresolved placeholders issue.

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

backend/prompt_executor.py

@@ -64,6 +64,8 @@ def validate_json_output(output: str, schema: Optional[Dict] = None, debug_info:
     """
     Validate that output is valid JSON.
 
+    Unwraps Markdown-wrapped JSON (```json ... ```) if present.
+
     Args:
         output: String to validate
         schema: Optional JSON schema to validate against (TODO: jsonschema library)
@@ -75,14 +77,28 @@ def validate_json_output(output: str, schema: Optional[Dict] = None, debug_info:
     Raises:
         HTTPException: If output is not valid JSON (with debug info attached)
     """
+    # Try to unwrap Markdown code blocks (common AI pattern)
+    unwrapped = output.strip()
+    if unwrapped.startswith('```json'):
+        # Extract content between ```json and ```
+        lines = unwrapped.split('\n')
+        if len(lines) > 2 and lines[-1].strip() == '```':
+            unwrapped = '\n'.join(lines[1:-1])
+    elif unwrapped.startswith('```'):
+        # Generic code block
+        lines = unwrapped.split('\n')
+        if len(lines) > 2 and lines[-1].strip() == '```':
+            unwrapped = '\n'.join(lines[1:-1])
+
     try:
-        parsed = json.loads(output)
+        parsed = json.loads(unwrapped)
         # TODO: Add jsonschema validation if schema provided
         return parsed
     except json.JSONDecodeError as e:
         error_detail = {
             "error": f"AI returned invalid JSON: {str(e)}",
             "raw_output": output[:500] + ('...' if len(output) > 500 else ''),
+            "unwrapped": unwrapped[:500] if unwrapped != output else None,
             "output_length": len(output)
         }
         if debug_info:
@@ -352,6 +368,7 @@ async def execute_prompt_with_data(
         Execution result dict
     """
     from datetime import datetime, timedelta
+    from placeholder_resolver import get_placeholder_example_values
 
     # Build variables from data modules
     variables = {
@@ -359,6 +376,16 @@ async def execute_prompt_with_data(
         'today': datetime.now().strftime('%Y-%m-%d')
     }
 
+    # Add PROCESSED placeholders (name, weight_trend, caliper_summary, etc.)
+    # This makes old-style prompts work with the new executor
+    try:
+        processed_placeholders = get_placeholder_example_values(profile_id)
+        variables.update(processed_placeholders)
+    except Exception as e:
+        # Continue even if placeholder resolution fails
+        if enable_debug:
+            variables['_placeholder_error'] = str(e)
+
     # Load data for enabled modules
     if modules:
         with get_db() as conn: