feat: add server foundation (config, database, auth, main)

- config: Load from env vars (SERVER_PORT, DATABASE_URL, JWT_SECRET, CAS_DIR, etc.)
- database: pgx/v5 connection pool, models (User, YggdrasilSession, Modpack, GlobalFile, LauncherRelease)
- auth: Yggdrasil endpoints (authenticate, refresh, validate) with SHA-256 password hashing, token rotation
- main: graceful shutdown, HTTP server on configured port
- go.mod: module gitea.mrixs.me/Mrixs/MrixsCraft-server, pgx/v5 dependency

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

internal/database/database.go

@@ -1,2 +1,87 @@
-// package database manages PostgreSQL connections, migrations, and data models.
+// package database manages PostgreSQL connections and data models.
 package database
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// DB wraps pgxpool.Pool with application-specific helpers.
+type DB struct {
+	pool *pgxpool.Pool
+}
+
+// Open creates a new database connection pool.
+func Open(ctx context.Context, databaseURL string) (*DB, error) {
+	pool, err := pgxpool.New(ctx, databaseURL)
+	if err != nil {
+		return nil, fmt.Errorf("connecting to database: %w", err)
+	}
+	if err := pool.Ping(ctx); err != nil {
+		pool.Close()
+		return nil, fmt.Errorf("pinging database: %w", err)
+	}
+	return &DB{pool: pool}, nil
+}
+
+// Close shuts down the connection pool.
+func (db *DB) Close() {
+	db.pool.Close()
+}
+
+// Pool returns the underlying pool for direct queries.
+func (db *DB) Pool() *pgxpool.Pool {
+	return db.pool
+}
+
+// ── Models ─────────────────────────────────────────────────────
+
+// User represents a registered player.
+type User struct {
+	ID           int    `db:"id"`
+	Username     string `db:"username"`
+	Email        string `db:"email"`
+	PasswordHash string `db:"password_hash"`
+	UUID         string `db:"uuid"`
+	Role         string `db:"role"`
+}
+
+// YggdrasilSession represents an active authentication session.
+type YggdrasilSession struct {
+	ClientToken string `db:"client_token"`
+	AccessToken string `db:"access_token"`
+	UserID      int    `db:"user_id"`
+	ExpiresAt   string `db:"expires_at"`
+}
+
+// Modpack represents a game server / modpack configuration.
+type Modpack struct {
+	ID               int    `db:"id"`
+	Slug             string `db:"slug"`
+	Name             string `db:"name"`
+	MinecraftVersion string `db:"minecraft_version"`
+	JavaVersion      int    `db:"java_version"`
+	ServerIP         string `db:"server_ip"`
+	IsActive         bool   `db:"is_active"`
+}
+
+// GlobalFile is a CAS entry.
+type GlobalFile struct {
+	SHA1     string `db:"sha1"`
+	Size     int64  `db:"size_bytes"`
+	FileName string `db:"file_name"`
+}
+
+// LauncherRelease tracks published launcher binaries.
+type LauncherRelease struct {
+	ID         int    `db:"id"`
+	Version    string `db:"version"`
+	OS         string `db:"os"`
+	Arch       string `db:"arch"`
+	SHA256     string `db:"sha256"`
+	FilePath   string `db:"file_path"`
+	IsActive   bool   `db:"is_active"`
+	IsMandatory bool  `db:"is_mandatory"`
+}