feat: add admin publishing workflow and yar theme

Add Go/Postgres admin APIs, Angular admin UI, manual build flow, asset uploads, markdown import/export, configurable slug generation, and the Yar reading theme. Exclude local docs and generated development artifacts from version control.

backend/internal/admin/status.go

@@ -0,0 +1,72 @@
+package admin
+
+import "fmt"
+
+func ValidPostStatus(status PostStatus) bool {
+	switch status {
+	case PostStatusDraft, PostStatusPublished, PostStatusArchived, PostStatusDeleted:
+		return true
+	default:
+		return false
+	}
+}
+
+func IsPublicPostStatus(status PostStatus) bool {
+	return status == PostStatusPublished || status == PostStatusArchived
+}
+
+func ValidVersionReason(reason VersionReason) bool {
+	switch reason {
+	case VersionReasonSave, VersionReasonPublish, VersionReasonUnpublish, VersionReasonArchive, VersionReasonRestore, VersionReasonImport, VersionReasonRollback:
+		return true
+	default:
+		return false
+	}
+}
+
+func ValidBuildJobStatus(status BuildJobStatus) bool {
+	switch status {
+	case BuildJobStatusQueued, BuildJobStatusRunning, BuildJobStatusSuccess, BuildJobStatusFailed, BuildJobStatusCancelled:
+		return true
+	default:
+		return false
+	}
+}
+
+func ValidBuildJobTrigger(trigger BuildJobTrigger) bool {
+	switch trigger {
+	case BuildJobTriggerPublish, BuildJobTriggerManual, BuildJobTriggerImport, BuildJobTriggerSync:
+		return true
+	default:
+		return false
+	}
+}
+
+func CanTransitionPostStatus(from PostStatus, to PostStatus) bool {
+	if !ValidPostStatus(from) || !ValidPostStatus(to) {
+		return false
+	}
+	if from == to {
+		return true
+	}
+
+	switch from {
+	case PostStatusDraft:
+		return to == PostStatusPublished || to == PostStatusArchived || to == PostStatusDeleted
+	case PostStatusPublished:
+		return to == PostStatusDraft || to == PostStatusArchived || to == PostStatusDeleted
+	case PostStatusArchived:
+		return to == PostStatusDraft || to == PostStatusPublished || to == PostStatusDeleted
+	case PostStatusDeleted:
+		return to == PostStatusDraft
+	default:
+		return false
+	}
+}
+
+func ValidatePostStatusTransition(from PostStatus, to PostStatus) error {
+	if CanTransitionPostStatus(from, to) {
+		return nil
+	}
+	return fmt.Errorf("invalid post status transition: %s -> %s", from, to)
+}