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/store/migrate_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"database/sql"
 	"errors"
+	"fmt"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -402,3 +403,67 @@ func TestMigration10MergesCPVDuplicates(t *testing.T) {
 		t.Error("a second row for a CPV code was accepted")
 	}
 }
+
+// Migration 11 indexes the description, buyer name and classification
+// descriptions of rows already stored, and its Down leaves the title
+// index working.
+func TestMigration11IndexesStoredRows(t *testing.T) {
+	ctx := context.Background()
+	m, db := newMigrator(t)
+	if _, err := m.UpTo(ctx, 10); err != nil {
+		t.Fatal(err)
+	}
+
+	setup := []string{
+		`INSERT INTO opportunities (uuid, uuid_type, title, status,
+			description, buyer_name, latest_release_date)
+		 VALUES ('a', 'ocid', 'Cloud', 'active', 'Servers', 'Leeds',
+			'2026-10-01')`,
+		`INSERT INTO classification_codes (id, is_cpv, code, classification)
+		 VALUES (1, 1, '15800000', 'Miscellaneous food products')`,
+	}
+	for _, statement := range setup {
+		if _, err := db.ExecContext(ctx, statement); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	if _, err := m.UpTo(ctx, 11); err != nil {
+		t.Fatal(err)
+	}
+
+	count := func(query string) int {
+		t.Helper()
+		var n int
+		if err := db.QueryRowContext(ctx, query).Scan(&n); err != nil {
+			t.Fatalf("%s: %v", query, err)
+		}
+		return n
+	}
+	matches := []struct {
+		table, match string
+		want         int
+	}{
+		{"opportunities_fts", "description : servers", 1},
+		{"opportunities_fts", "buyer_name : leeds", 1},
+		{"opportunities_fts", "title : servers", 0},
+		{"classification_codes_fts", "food", 1},
+	}
+	for _, tc := range matches {
+		got := count(fmt.Sprintf(
+			"SELECT count(*) FROM %s WHERE %[1]s MATCH '%s'", tc.table, tc.match,
+		))
+		if got != tc.want {
+			t.Errorf("%s MATCH %q: %d; want %d", tc.table, tc.match, got, tc.want)
+		}
+	}
+
+	if _, err := m.DownTo(ctx, 10); err != nil {
+		t.Fatal(err)
+	}
+	got := count(`SELECT count(*) FROM opportunities_fts
+		WHERE opportunities_fts MATCH 'cloud'`)
+	if got != 1 {
+		t.Errorf("title search after Down: %d; want 1", got)
+	}
+}