feat: add API handler (register, login, skin, launcher, servers, manifest)

- register: POST /api/web/register — create user with SHA-256 password hash
- login: POST /api/web/login — credentials check + session token
- uploadSkin: POST /api/web/profile/skin — PNG upload, SHA-1 CAS storage
- launcherLatest: GET /api/launcher/latest — latest launcher version + download URL
- serversList: GET /api/servers.json — active modpacks list
- instanceManifest: GET /api/instances/{slug}/manifest.json — modpack manifest
- serveSkin: GET /skins/{hash}.png — skin file serving with cache headers
- PathValue-based routing (Go 1.22+)

Co-Authored-By: OWL <noreply@anthropic.com>

cmd/server/main.go

@@ -10,6 +10,7 @@ import (
 	"syscall"
 	"time"
 
+	"gitea.mrixs.me/Mrixs/MrixsCraft-server/internal/api"
 	"gitea.mrixs.me/Mrixs/MrixsCraft-server/internal/auth"
 	"gitea.mrixs.me/Mrixs/MrixsCraft-server/internal/config"
 	"gitea.mrixs.me/Mrixs/MrixsCraft-server/internal/database"
@@ -41,7 +42,11 @@ func main() {
 	authHandler := auth.NewHandler(db, cfg)
 	authHandler.RegisterRoutes(mux)
 
-	// TODO: register API, Admin, CAS routes.
+	// Public API.
+	apiHandler := api.NewHandler(db, cfg)
+	apiHandler.RegisterRoutes(mux)
+
+	// TODO: register Admin, CAS routes.
 
 	addr := ":" + itoa(cfg.Port)
 	srv := &http.Server{