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/chat.py

@@ -6,7 +6,7 @@ import httpx
 from quart import Blueprint, Response, jsonify, request
 
 from fabledassistant.auth import login_required, get_current_user_id
-from fabledassistant.routes.utils import not_found
+from fabledassistant.routes.utils import not_found, parse_pagination
 from fabledassistant.config import Config
 from fabledassistant.services.chat import (
     add_message,
@@ -38,8 +38,7 @@ chat_bp = Blueprint("chat", __name__, url_prefix="/api/chat")
 @login_required
 async def list_conversations_route():
     uid = get_current_user_id()
-    limit = min(request.args.get("limit", 50, type=int), 500)
-    offset = request.args.get("offset", 0, type=int)
+    limit, offset = parse_pagination()
     conv_type = request.args.get("type", "chat")
     # Apply retention policy before returning list
     retention_str = await get_setting(uid, "chat_retention_days", "90")