feat(docker-watcher): phase 12 - hardening

Blue-green zero-downtime deploys, promote flow validation.
Dual auth: local (bcrypt + JWT) and OAuth2/OIDC (any provider).
Auth middleware, login page, auth settings UI.
Structured logging (slog JSON), config export to YAML.
Graceful shutdown with deploy draining.
Multi-stage Dockerfile and production docker-compose.yml.
Swap phase order: Volumes & Env before UI Polish.

internal/api/response.go

@@ -2,7 +2,7 @@ package api
 
 import (
 	"encoding/json"
-	"log"
+	"log/slog"
 	"net/http"
 )
 
@@ -18,7 +18,7 @@ func respondJSON(w http.ResponseWriter, status int, data any) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)
 	if err := json.NewEncoder(w).Encode(envelope{Success: true, Data: data}); err != nil {
-		log.Printf("[api] encode response: %v", err)
+		slog.Error("encode response", "error", err)
 	}
 }
 
@@ -27,7 +27,7 @@ func respondError(w http.ResponseWriter, status int, msg string) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)
 	if err := json.NewEncoder(w).Encode(envelope{Success: false, Error: msg}); err != nil {
-		log.Printf("[api] encode error response: %v", err)
+		slog.Error("encode error response", "error", err)
 	}
 }