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.

internal/store/README.md

@@ -110,7 +110,7 @@ ts, err := s.ListOpportunities(ctx, store.OpportunityFilter{
 })
 rs, err := s.ListReleases(ctx, uuid) // oldest first, with tags
 batch, err := s.LatestChangeBatch(ctx) // hash of the newest change batch, or ""
-ts, batch, err = s.ListChangedOpportunities(ctx, batch) // changed since; ErrUnknownBatch
+ts, batch, err = s.ListChangedOpportunities(ctx, batch, time.Time{}) // changed since; ErrUnknownBatch
 ok, err := s.ReleaseExists(ctx, uuid, releaseID)
 ```
 
@@ -127,7 +127,7 @@ ok, err := s.ReleaseExists(ctx, uuid, releaseID)
 
 **Listing.** `ListOpportunities` filters by status, classification code, title text, a range on `LatestReleaseDate` and each mark (`MarkSet` or `MarkUnset`; an opportunity with no marks stored has every mark unset), newest first, 50 per page by default. It loads classifications and marks but not releases (which can be large); those are loaded by `ListReleases` or `GetOpportunity`.
 
-`LatestChangeBatch` returns the hash of the newest change batch ("" if there is none). Read before listing, it is the batch to give `ListChangedOpportunities`, which returns the opportunities in every later batch, once each, unpaged and ordered as `ListOpportunities` orders them, with the hash of the newest of those batches to give it next time. Batches recorded while it runs are left for next time rather than missed. A hash it doesn't know, such as a deleted batch's, is `ErrUnknownBatch`.
+`LatestChangeBatch` returns the hash of the newest change batch ("" if there is none). Read before listing, it is the batch to give `ListChangedOpportunities`, which returns the opportunities in every later batch, once each, unpaged and ordered as `ListOpportunities` orders them, with the hash of the newest of those batches to give it next time. A non-zero `releasedFrom` leaves out opportunities whose latest release is before it, as `OpportunityFilter.ReleasedFrom` does; the hash is the same either way. Batches recorded while it runs are left for next time rather than missed. A hash it doesn't know, such as a deleted batch's, is `ErrUnknownBatch`.
 
 
 **Title search.** The words are matched against the title using FTS5: every word must appear, each also matches as a prefix, case and accents are ignored, and "migrating" finds "migration", etc. Input is sanitised, so FTS5 operators and quotes typed by a user are searched as plain words, never interpreted.