add todo about the server cache

TODO.md

@@ -58,6 +58,28 @@ exported as a public constant. Callers currently pass the default.
 `default_risk_free_rate` in `src/analytics/risk.zig`. Eventually consider
 making this a config value (env var or .env) so it doesn't require a rebuild.
 
+## On-demand server-side fetch for new symbols
+
+Currently the server's SRF endpoints (`/candles`, `/dividends`, etc.) are pure
+cache reads — they 404 if the data isn't already on disk. New symbols only get
+populated when added to the portfolio and picked up by the next cron refresh.
+
+Consider: on a cache miss, instead of blocking the HTTP response with a
+multi-second provider fetch, kick off an async background fetch (or just
+auto-add the symbol to the portfolio) and return 404 as usual. The next
+request — or the next cron run — would then have the data. This gives
+"instant-ish gratification" for new symbols without the downsides of
+synchronous fetch-on-miss (latency, rate limit contention, unbounded cache
+growth from arbitrary tickers).
+
+Note that this process doesn't do anything to eliminate all the API keys
+that are necessary for a fully functioning system. A more aggressive view
+would be to treat ZFIN_SERVER has a 100% record of reference, but that would
+introduce some opacity to the process as we wait for candles (for example) to
+populate. This could be solved on the server by spawning a thread to fetch the
+data, then returning 202 Accepted, which could then be polled client side. Maybe
+this is a better long term approach?
+
 ## CLI/TUI code review (lower priority)
 
 No review has been done on these files. They are presentation-layer code