Factsheet URL field on holdings

Add an optional factsheet / document URL field to fund holdings, so users can attach the official factsheet (or a self-hosted Dropbox/Drive copy) and open it in one click from the platform.

Use case: when reviewing a portfolio once or twice a year, or when evaluating a new fund against an existing position, quickly opening the factsheet for side-by-side comparison.

Proposed scope:
- One URL per holding (per-user, per-ISIN), works for funds/ETFs/stocks/bonds/custom assets.
- Settable from the add-holding and edit-holding forms.
- Surfaced as a link on the stock details page and in the allocations drill-down rows (where users compare positions).
- Self-hosting supported -- Dropbox / Drive / OneDrive URLs accepted, in case the issuer changes their published URL.