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/projects.py

@@ -4,7 +4,7 @@ import logging
 from quart import Blueprint, 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.services.milestones import list_milestones
 from fabledassistant.services.notes import list_notes
 from fabledassistant.services.projects import (
@@ -100,8 +100,7 @@ async def get_project_notes_route(project_id: int):
     # type filter: "note", "task", or None (both)
     type_filter = request.args.get("type")
     status_filter = request.args.get("status")
-    limit = min(request.args.get("limit", 100, type=int), 500)
-    offset = request.args.get("offset", 0, type=int)
+    limit, offset = parse_pagination(default_limit=100)
 
     is_task: bool | None = None
     if type_filter == "task":