Replaced the Title filter with a general search, run by the backend

GET /api/opportunities/search?q= returns the UUIDs of the matching
opportunities, and the frontend filters its list by them. Words without
a prompt search the title and description; title:, buyer:, buyer_id:
and cpv: search only their own field, and a leading ! leaves out what a
term matches. Repeats of one prompt are OR, everything else AND. CPV
terms are checked per code: cpv:72 !cpv:7221 needs a code starting 72
that doesn't start 7221.

Unknown prompts are reported, and every response carries the help that
the search box's "?" panel shows. Migration 00011 adds the description
and buyer name to the full-text index, and indexes CPV descriptions.

internal/api/opportunities_test.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"path/filepath"
+	"slices"
 	"strings"
 	"sync"
 	"testing"
@@ -483,3 +484,54 @@ func TestChangeMarkPublishes(t *testing.T) {
 		}
 	}
 }
+
+// GET /opportunities/search returns the matching UUIDs, bounded by
+// released_from, with the help; a search with nothing to search for matches
+// all, and unknown prompts are listed.
+func TestSearchOpportunities(t *testing.T) {
+	url := opportunitiesServer(t, 5) + "/opportunities/search"
+	from3 := "&released_from=2026-10-03T00:00:00Z"
+	cases := []struct {
+		query      string
+		uuids      []string
+		matchesAll bool
+		unknown    []string
+	}{
+		{"", []string{}, true, []string{}},
+		{"?q=", []string{}, true, []string{}},
+		{"?q=cpv:72", []string{"ocds-1", "ocds-3", "ocds-5"}, false, []string{}},
+		{"?q=cpv:72" + from3, []string{"ocds-3", "ocds-5"}, false, []string{}},
+		{"?q=opportunity%20!cpv:72", []string{"ocds-2", "ocds-4"}, false, []string{}},
+		{"?q=nothing", []string{}, false, []string{}},
+		{"?q=foo:x%20Bar:y", []string{}, true, []string{"foo", "Bar"}},
+		{"?q=foo:x%20title:3", []string{"ocds-3"}, false, []string{"foo"}},
+	}
+	for _, tc := range cases {
+		var got OpportunitySearch
+		status := request(t, http.MethodGet, url+tc.query, &got)
+		slices.Sort(got.UUIDs)
+		if status != http.StatusOK || !slices.Equal(got.UUIDs, tc.uuids) ||
+			got.MatchesAll != tc.matchesAll ||
+			!slices.Equal(got.UnknownPrompts, tc.unknown) {
+			t.Errorf("%s: status %d, %+v; want %v, matches_all %v, unknown %v",
+				tc.query, status, got, tc.uuids, tc.matchesAll, tc.unknown)
+		}
+		if len(got.Prompts) == 0 || len(got.Syntax) == 0 {
+			t.Errorf("%s: no help: %+v", tc.query, got)
+		}
+	}
+
+	// Empty lists are [], never null.
+	var raw map[string]json.RawMessage
+	request(t, http.MethodGet, url+"?q=nothing", &raw)
+	for _, field := range []string{"uuids", "unknown_prompts"} {
+		if string(raw[field]) != "[]" {
+			t.Errorf("%s = %s; want []", field, raw[field])
+		}
+	}
+
+	status := request(t, http.MethodGet, url+"?q=x&released_from=x", nil)
+	if status != http.StatusBadRequest {
+		t.Errorf("bad released_from: status %d; want 400", status)
+	}
+}