Added a "Published in the last X days" filter, applied by the backend

GET /api/opportunities takes released_from, an RFC 3339 time, and lists
only the opportunities whose latest release is at or after it. It works
with paging and with changed_after, which keeps the same batch either
way; an invalid value is 400.

The frontend's filter defaults to 7 days, from midnight UK time, and
sends it with every load and catch-up, so a year of opportunities isn't
downloaded to show a week's. Changing the days loads the list again;
Clear filters goes back to 7.

web/README.md

@@ -36,6 +36,7 @@ The frontend uses, through `src/api/client.ts`:
 | ------ | ------------------------------------------ | ---------------------------------------------- |
 | GET    | `/api/opportunities?offset=&limit=`        | A page of opportunities, with their marks      |
 | GET    | `/api/opportunities?changed_after={batch}` | The opportunities changed since a change batch |
+| GET    | `/api/opportunities/{uuid}`                | One opportunity, with its releases             |
 | GET    | `/api/classifications`                     | Every classification in use, by code           |
 | PUT    | `/api/opportunities/{uuid}/marks/{mark}`   | Sets one mark; returns all the marks           |
 | DELETE | `/api/opportunities/{uuid}/marks/{mark}`   | Clears one mark; returns all the marks         |
@@ -61,6 +62,11 @@ the batch it's up to date with, and asks for everything changed after it
 their marks changed. If the backend doesn't know the batch (410), or has
 recorded none yet, the whole list is reloaded instead.
 
+Both list requests send `released_from` (an RFC 3339 time), so only the
+opportunities published in the last few days the filter names (7 by default,
+from midnight UK time) are loaded; changing the number of days loads the list
+again.
+
 The list is fetched a page (100) at a time until there are no more. The marks
 are `interested`, `skipped`, `bid_submitted`, `lost`, `won` and `hidden`;
 Won and Lost exclude each other (409). The classifications are the ones the