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/tasks.py

@@ -4,7 +4,7 @@ from quart import Blueprint, jsonify, request
 
 from fabledassistant.auth import login_required, get_current_user_id
 from fabledassistant.models.note import TaskPriority, TaskStatus
-from fabledassistant.routes.utils import not_found, parse_iso_date
+from fabledassistant.routes.utils import not_found, parse_iso_date, parse_pagination
 from fabledassistant.services.embeddings import upsert_note_embedding
 from fabledassistant.services.notes import (
     create_note,
@@ -28,8 +28,7 @@ async def list_tasks_route():
     priority = request.args.get("priority")
     sort = request.args.get("sort", "updated_at")
     order = request.args.get("order", "desc")
-    limit = min(request.args.get("limit", 50, type=int), 500)
-    offset = request.args.get("offset", 0, type=int)
+    limit, offset = parse_pagination()
 
     project_id = request.args.get("project_id", type=int)
     no_project = request.args.get("no_project", "").lower() == "true"