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/config.go

@@ -0,0 +1,60 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strconv"
+)
+
+// Config aggregates every runtime knob the sidecar reads from the
+// environment. Held in one struct so the rest of the package can take a
+// pointer instead of poking os.Getenv at use-sites.
+type Config struct {
+	OriginalsRoot     string // absolute path to PhotoPrism's originals dir
+	PhotoprismBaseURL string // e.g. http://localhost:2342
+	Port              int    // HTTP listen port (loopback only)
+	DSN               string // GORM/MySQL connection string for mule_sidecar
+}
+
+func loadConfig() (*Config, error) {
+	root := envOr("ORIGINALS_ROOT", "/photoprism/originals")
+	abs, err := filepath.Abs(root)
+	if err != nil {
+		return nil, err
+	}
+
+	portStr := envOr("SIDECAR_PORT", "8000")
+	port, err := strconv.Atoi(portStr)
+	if err != nil {
+		return nil, err
+	}
+
+	dsn := os.Getenv("SIDECAR_DSN")
+	if dsn == "" {
+		// Default matches the user that mariadb/init/01-sidecar.sql provisions
+		// on first boot. The literal placeholder password is intentional: the
+		// SQL ships with it and an env-templating step is left for whoever
+		// runs this in a non-local context.
+		user := envOr("SIDECAR_DB_USER", "sidecar")
+		pass := envOr("SIDECAR_DB_PASSWORD", "replace-at-m4-bringup")
+		host := envOr("SIDECAR_DB_HOST", "127.0.0.1")
+		dbPort := envOr("SIDECAR_DB_PORT", "3306")
+		name := envOr("SIDECAR_DB_NAME", "mule_sidecar")
+		dsn = user + ":" + pass + "@tcp(" + host + ":" + dbPort + ")/" + name +
+			"?charset=utf8mb4&parseTime=true&loc=Local"
+	}
+
+	return &Config{
+		OriginalsRoot:     abs,
+		PhotoprismBaseURL: envOr("PHOTOPRISM_BASE_URL", "http://localhost:2342"),
+		Port:              port,
+		DSN:               dsn,
+	}, nil
+}
+
+func envOr(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}