refactor: DRY pass on backend — pagination helper and sharing utilities

- Add parse_pagination() to routes/utils.py; replace 6 duplicate limit/offset extractions in notes, tasks, chat, projects, milestones routes
- Extract _enrich_shares() in sharing.py; eliminates identical 12-line loop in list_project_shares and list_note_shares
- Extract _deduplicate_by_permission() in sharing.py; eliminates identical deduplication blocks in list_shared_with_me for projects and notes

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

src/fabledassistant/routes/utils.py

@@ -1,6 +1,6 @@
 from datetime import date
 
-from quart import jsonify
+from quart import jsonify, request
 
 
 def not_found(resource: str = "Item"):
@@ -15,3 +15,10 @@ def parse_iso_date(value: str | None, field: str = "date"):
         return date.fromisoformat(value)
     except ValueError:
         return jsonify({"error": f"Invalid {field} format. Use YYYY-MM-DD."}), 400
+
+
+def parse_pagination(default_limit: int = 50, max_limit: int = 500) -> tuple[int, int]:
+    """Extract and clamp ``limit`` / ``offset`` from the current request's query string."""
+    limit = min(request.args.get("limit", default_limit, type=int), max_limit)
+    offset = request.args.get("offset", 0, type=int)
+    return limit, offset