Fix image search: embed markdown instead of describing URLs

- tools.py: search_images result now includes 'embed' (ready-to-use
  markdown image syntax) and 'citation' fields instead of raw 'local_url';
  adds 'instructions' field so the model knows to render them verbatim
- llm.py: system prompt now explicitly tells the model to embed images
  using the 'embed' field rather than describing or listing URLs
- markdown.ts: explicitly allow src/alt in PURIFY_OPTS_FULL so img tags
  are never stripped by DOMPurify

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

src/fabledassistant/services/tools.py

@@ -1264,18 +1264,28 @@ async def execute_tool(user_id: int, tool_name: str, arguments: dict) -> dict:
                     source_domain=r.get("source_domain"),
                 )
                 if record:
+                    title = record.title or query
+                    page_url = r.get("page_url", "")
+                    source_domain = r.get("source_domain", "")
                     images.append({
-                        "local_url": f"/api/images/{record.id}",
-                        "page_url": r.get("page_url", ""),
-                        "source_domain": r.get("source_domain", ""),
-                        "title": record.title or query,
+                        "embed": f"![{title}](/api/images/{record.id})",
+                        "citation": f"*Source: [{source_domain or 'image'}]({page_url})*",
+                        "page_url": page_url,
+                        "title": title,
                     })
             if not images:
                 return {"success": False, "error": f"Could not retrieve images for '{query}'"}
             return {
                 "success": True,
                 "type": "image_search",
-                "data": {"query": query, "images": images},
+                "data": {
+                    "query": query,
+                    "images": images,
+                    "instructions": (
+                        "Embed each image in your response by writing the 'embed' field verbatim, "
+                        "then the 'citation' field on the next line. Do not paraphrase or list URLs."
+                    ),
+                },
             }
 
         else: