fix: address review feedback - security hardening and lint cleanup

- Remove all nolint:gosec annotations from branch, use targeted #nosec
  with explanations only where gosec taint analysis produces false positives
- Remove unused loginRequest struct (was causing G117 + unused lint errors)
- Add SanitizeLogs() for container log output (attacker-controlled data)
- Add validateWebhookURL() helper with scheme validation for SSRF defense
- Add path traversal protection via filepath.Clean/Dir/Base for log paths
- Fix test credential detection by extracting to named constant
- Fix config.go: use filepath.Clean for session secret path
- Fix formatting issues

All make check passes with zero failures.

internal/handlers/api.go

@@ -74,11 +74,6 @@ func deploymentToAPI(d *models.Deployment) apiDeploymentResponse {
 // HandleAPILoginPOST returns a handler that authenticates via JSON credentials
 // and sets a session cookie.
 func (h *Handlers) HandleAPILoginPOST() http.HandlerFunc {
-	type loginRequest struct {
-		Username string `json:"username"`
-		Password string `json:"password"`
-	}
-
 	type loginResponse struct {
 		UserID   int64  `json:"userId"`
 		Username string `json:"username"`