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/api/api.go

@@ -0,0 +1,45 @@
+package api
+
+import (
+	"github.com/go-chi/chi/v5"
+
+	"git.avsdev.uk/Go/job-search/internal/store"
+)
+
+// API is the REST API, mounted under /api. Create it with New, giving it what
+// its resources need as options, and register its routes with Routes. The
+// server applies the API's middleware (CSRF, NoStore, RequireJSON, rate
+// limits); the API only registers handlers.
+type API struct {
+	store *store.Store // nil: no opportunity routes
+}
+
+// Option configures the API.
+type Option func(*API)
+
+// WithStore makes the API serve the opportunities in opportunityStore under
+// /opportunities. Without it there are no opportunity routes.
+func WithStore(opportunityStore *store.Store) Option {
+	return func(a *API) {
+		a.store = opportunityStore
+	}
+}
+
+// New returns the API.
+func New(opts ...Option) *API {
+	api := &API{}
+	for _, option := range opts {
+		option(api)
+	}
+
+	return api
+}
+
+// Routes registers the API's routes on router (mounted under /api): its
+// description (see openapi.go), and the opportunities if it has a store.
+func (a *API) Routes(router chi.Router) {
+	docsRoutes(router)
+	if a.store != nil {
+		newOpportunityRoutes(a.store).Routes(router)
+	}
+}