When a holding has no price (ISIN not in the built-in map and Yahoo
rejects the raw ISIN), the portfolio page now shows an amber banner
listing each missing ISIN with an inline text input and a "Look up"
link to Yahoo Finance symbol search.
Submitting the form POSTs to /portfolio/ticker which upserts the mapping
into a finance_ticker_mappings collection keyed by (user_id, ISIN).
On the next page load custom mappings are resolved first, before the
hardcoded isinToTicker table, so user overrides always win.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
