fix: golangci-lint v2 config and lint-clean production code

- Fix .golangci.yml for v2 format (linters-settings -> linters.settings)
- All production code now passes golangci-lint with zero issues
- Line length 88, funlen 80/50, cyclop 15, dupl 100
- Extract shared helpers in db (scanChannels, scanInt64s, scanMessages)
- Split runMigrations into applyMigration/execMigration
- Fix fanOut return signature (remove unused int64)
- Add fanOutSilent helper to avoid dogsled
- Rewrite CLI code for lint compliance (nlreturn, wsl_v5, noctx, etc)
- Rename CLI api package to chatapi to avoid revive var-naming
- Fix all noinlineerr, mnd, perfsprint, funcorder issues
- Fix db tests: extract helpers, add t.Parallel, proper error checks
- Broker tests already clean
- Handler integration tests still have lint issues (next commit)

internal/db/export_test.go

@@ -0,0 +1,47 @@
+package db
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"log/slog"
+	"sync/atomic"
+)
+
+//nolint:gochecknoglobals // test counter
+var testDBCounter atomic.Int64
+
+// NewTestDatabase creates an in-memory database for testing.
+func NewTestDatabase() (*Database, error) {
+	n := testDBCounter.Add(1)
+
+	dsn := fmt.Sprintf(
+		"file:testdb%d?mode=memory"+
+			"&cache=shared&_pragma=foreign_keys(1)",
+		n,
+	)
+
+	d, err := sql.Open("sqlite", dsn)
+	if err != nil {
+		return nil, err
+	}
+
+	database := &Database{db: d, log: slog.Default()}
+
+	err = database.runMigrations(context.Background())
+	if err != nil {
+		closeErr := d.Close()
+		if closeErr != nil {
+			return nil, closeErr
+		}
+
+		return nil, err
+	}
+
+	return database, nil
+}
+
+// Close closes the underlying database connection.
+func (s *Database) Close() error {
+	return s.db.Close()
+}