feat(sharing): add document sharing with expiring links

- Add SharedLink model with token, expiry, view limit, password hash
- Add migration 025_add_shared_links
- Add API endpoints: create, list, revoke (auth) + public info/download
- Add management UI at /shared-links with revoke controls
- Add public share landing page at /share/{token}
- Add Share button on file_view.html
- Add Shared Links to user dropdown in common.js
- Write 35 unit tests covering all scenarios
- Update UserGuide.md with sharing documentation

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/views/share.py

@@ -0,0 +1,28 @@
+"""Public view for accessing a shared document link.
+
+This route does NOT require authentication — it is the landing page
+that link recipients visit.  The page fetches link metadata via the
+public ``/api/share/{token}/info`` JSON endpoint and then renders the
+appropriate download UI (password gate or direct download button).
+"""
+
+import logging
+import pathlib
+
+from fastapi import APIRouter, Request
+from fastapi.templating import Jinja2Templates
+
+logger = logging.getLogger(__name__)
+router = APIRouter()
+
+_templates_dir = pathlib.Path(__file__).parents[2] / "frontend" / "templates"
+templates = Jinja2Templates(directory=str(_templates_dir))
+
+
+@router.get("/share/{token}")
+async def shared_link_view(request: Request, token: str):
+    """Render the public share landing page for a given token."""
+    return templates.TemplateResponse(
+        "shared_link_view.html",
+        {"request": request, "token": token},
+    )