feat(sidecar): port Node prototype to Go + Gin + GORM + MariaDB

Replace the Node prototype (server.mjs) with the stack the merge plan
calls for: Go 1.25, Gin for routing, GORM + MariaDB for persistence.
Same wire contract on /api/sidecar/* so the SvelteKit client doesn't
change.

- Marks move from a JSON file on disk to mule_sidecar.marks (auto-
  migrated by GORM on first boot). The Node prototype's marks.json
  was dev-only; not migrated.
- Folder/rename/heap-convert/duplicates handlers reproduce the
  prototype's behaviour, including the path-traversal defence
  (resolveUnderRoot + EvalSymlinks), the size-bucket prefilter for
  the duplicate hasher, and the background reindex fire-and-forget
  pattern.
- Auth model unchanged: requireSession middleware proxies the
  caller's X-Auth-Token to PhotoPrism's /api/v1/photos?count=1
  before any destructive op.
- Expose pp-mariadb on 127.0.0.1:3306 in docker-compose so the
  host Go process can reach mule_sidecar.* without joining the
  container network.
- Archive the Node prototype under sidecar/legacy/server.mjs for
  one cycle as reference.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sidecar/db.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"time"
+
+	"gorm.io/driver/mysql"
+	"gorm.io/gorm"
+	"gorm.io/gorm/logger"
+)
+
+// Mark mirrors the per-photo extras the web client stores via the marks
+// endpoints — rating + four-colour label. PhotoUID is the row key; both
+// payload fields are nullable so the sparse "no rating / no colour" state
+// round-trips cleanly. The Node prototype kept this in a JSON file; we
+// migrate to MariaDB here so the M4 sharing work has a real table to
+// extend.
+type Mark struct {
+	PhotoUID  string    `gorm:"primaryKey;size:64;column:photo_uid" json:"-"`
+	Rating    *int      `gorm:"column:rating" json:"rating,omitempty"`
+	Color     *string   `gorm:"size:16;column:color" json:"color,omitempty"`
+	UpdatedAt time.Time `gorm:"column:updated_at" json:"updatedAt"`
+}
+
+// TableName pins the GORM-pluralised default to a name that matches the
+// other tables the M4 plan calls out (`marks`, `heap_shares`, …) so
+// nothing surprising lands in the schema.
+func (Mark) TableName() string { return "marks" }
+
+// asJSON returns the wire shape clients expect — same flat object the
+// Node prototype emitted. An empty Mark (rating=nil, color=nil) renders
+// as `{}` which the client treats as "no mark on this photo".
+func (m *Mark) asJSON() map[string]any {
+	out := map[string]any{}
+	if m == nil {
+		return out
+	}
+	if m.Rating != nil {
+		out["rating"] = *m.Rating
+	}
+	if m.Color != nil && *m.Color != "" {
+		out["color"] = *m.Color
+	}
+	if !m.UpdatedAt.IsZero() {
+		// ISO-8601 with millisecond precision, UTC — matches the Node
+		// prototype's `new Date().toISOString()` so clients written against
+		// the old endpoint stay happy.
+		out["updatedAt"] = m.UpdatedAt.UTC().Format("2006-01-02T15:04:05.000Z")
+	}
+	return out
+}
+
+func openDB(dsn string) (*gorm.DB, error) {
+	db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{
+		Logger: logger.Default.LogMode(logger.Warn),
+	})
+	if err != nil {
+		return nil, err
+	}
+	if err := db.AutoMigrate(&Mark{}); err != nil {
+		return nil, err
+	}
+	return db, nil
+}