Lint: fix real int-truncation bug; document intentional gosec findings

- TransportPool configKey: write conn-count ints verbatim (strconv) instead of
  byte() truncation — a config value >=256 previously truncated and made two
  distinct configs collide on one key, silently sharing a transport with the
  wrong settings (real fix, gosec G115).
- Document design-intentional gosec findings with //nolint:gosec + rationale:
  G117 (clients_secret intentionally marshaled into stored creds JSON),
  G704 (tokenURL is the trusted discovery endpoint), G124 (delete/test cookie,
  Secure set conditionally), G118 (long-lived background refresh) and pool
  utils short-ID rune, testutil mock server redirects.

Result: full-tree golangci-lint now reports 0 issues (was ~12). Build, vet,
and the full test suite remain green.

internal/dcrstorage/file.go

@@ -78,7 +78,7 @@ func (s *FileStore) Save(ctx context.Context, providerURL string, creds *ClientR
 		return fmt.Errorf("failed to create credentials directory: %w", err)
 	}
 
-	data, err := json.MarshalIndent(creds, "", "  ")
+	data, err := json.MarshalIndent(creds, "", "  ") //nolint:gosec // ClientSecret must be persisted; creds are the feature's output
 	if err != nil {
 		return fmt.Errorf("failed to marshal credentials: %w", err)
 	}

internal/dcrstorage/redis.go

@@ -86,7 +86,7 @@ func (s *RedisStore) Save(ctx context.Context, providerURL string, creds *Client
 	}
 
 	// Serialize credentials to JSON for storage
-	data, err := json.Marshal(creds)
+	data, err := json.Marshal(creds) //nolint:gosec // secret intentionally serialized for Redis storage
 	if err != nil {
 		return fmt.Errorf("failed to marshal credentials: %w", err)
 	}