Make artifact and media metadata first-class server records

Artifact and media metadata were previously reconstructed from job result
refs and JSON sidecar files. This change regularizes metadata into SQLite,
keeps filesystem blobs in place, and preserves backward compatibility via
lazy fallback and backfill from existing job refs and JSON metadata.

Constraint: Blob storage remains on the local filesystem in this phase
Rejected: Migrate blobs into SQLite | larger scope and worse operational profile for current media sizes
Rejected: Hard cutover without fallback | unsafe for historical data already on the test server
Confidence: medium
Scope-risk: moderate
Directive: Treat SQLite as the metadata source of truth; JSON sidecars are compatibility fallback only
Tested: go test ./...; deployed to test server 101.42.99.35; verified /v1/artifacts, /v1/artifacts/:id, signed media URL 200, unsigned content 401
Not-tested: Full historical backfill sweep over all existing artifact rows under production-sized data volume

internal/server/types.go

@@ -90,19 +90,26 @@ type logEntry struct {
 }
 
 type artifact struct {
-	ID            string    `json:"id"`
-	JobID         string    `json:"job_id"`
-	MediaID       string    `json:"media_id,omitempty"`
-	Filename      string    `json:"filename"`
-	ContentType   string    `json:"content_type"`
-	SizeBytes     int64     `json:"size_bytes"`
-	CreatedAt     string    `json:"created_at"`
-	ExpiresAt     string    `json:"expires_at"`
-	URL           string    `json:"url,omitempty"`
-	Visibility    string    `json:"visibility,omitempty"`
-	SHA256        string    `json:"sha256,omitempty"`
-	StorageStatus string    `json:"storage_status,omitempty"`
-	Ref           resultRef `json:"-"`
+	ID             string         `json:"id"`
+	JobID          string         `json:"job_id"`
+	ProjectID      string         `json:"project_id,omitempty"`
+	MediaID        string         `json:"media_id,omitempty"`
+	Filename       string         `json:"filename"`
+	ContentType    string         `json:"content_type"`
+	SizeBytes      int64          `json:"size_bytes"`
+	CreatedAt      string         `json:"created_at"`
+	ExpiresAt      string         `json:"expires_at"`
+	URL            string         `json:"url,omitempty"`
+	Visibility     string         `json:"visibility,omitempty"`
+	SHA256         string         `json:"sha256,omitempty"`
+	StorageStatus  string         `json:"storage_status,omitempty"`
+	SourceSkillID  string         `json:"source_skill_id,omitempty"`
+	SourceModelID  string         `json:"source_model_id,omitempty"`
+	SourceRouteKey string         `json:"source_route_key,omitempty"`
+	SourceInput    map[string]any `json:"source_input,omitempty"`
+	PromptText     string         `json:"prompt_text,omitempty"`
+	Usage          map[string]any `json:"usage,omitempty"`
+	Ref            resultRef      `json:"-"`
 }
 
 type media struct {