Added the opportunities REST API, read-only apart from their marks, described in OpenAPI 3.1 with swag

GET /api/opportunities lists the stored opportunities, newest release first,
a page at a time (offset, and limit up to 100, default 50); has_more says
whether another page follows. GET /api/opportunities/{uuid} returns one with
its releases. PUT and DELETE /api/opportunities/{uuid}/marks/{mark} set and
clear one mark (hidden, skipped, interested, bid_submitted, won, lost) and
return them all; won and lost exclude each other (409). The API uses the
types in internal/types, and changing a mark is made atomic, as the store
replaces all of an opportunity's marks at once. The marks are found by
reflection: every bool field of types.OpportunityMarks, by its JSON name, so
a new mark needs no API change.

The API has one entry point, api.New, with its dependencies as options
(api.WithStore), and the server only mounts it under /api, keeping the
middleware. New resources are added inside internal/api. job-search serve
passes server.WithStore when database.file is set, opening the store before
the server; without one there are no opportunity routes.

The API is described by swag v2 annotations on the handlers. go generate
writes the OpenAPI 3.1 description (git-ignored, like the OCDS types), which
the server embeds and serves at /api/openapi.json, with Swagger UI at
/api/docs. A test checks the description and the routes match. swag is in
the README's dev tools, pinned to v2.0.0-rc6, and the build now needs it.

internal/types/opportunities.go

@@ -58,7 +58,8 @@ var ErrWonAndLost = errors.New("types: an opportunity cannot be both won and los
 // OpportunityMarks are the user's own marks on an opportunity, kept apart
 // from the fetched data so saving a newer release never changes them. The
 // marks are independent except that Won and Lost exclude each other (see
-// Validate).
+// Validate). Every bool field is a mark, and the API sets and clears it by
+// its JSON name, so a field that isn't a mark mustn't be a bool.
 type OpportunityMarks struct {
 	UUID         string `db:"uuid"          json:"uuid"`
 	Hidden       bool   `db:"hidden"        json:"hidden"`