feat(internal): add platform detection and auth packages

- internal/platform: cross-platform path detection, 6 tests
- internal/auth: JWT generation/validation + bcrypt, 8 tests
- 89 total tests, build + vet clean

backend/internal/platform/platform.go

@@ -0,0 +1,141 @@
+// Package platform detects the runtime OS and environment,
+// providing platform-specific paths and configuration defaults.
+package platform
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+)
+
+// Platform holds information about the current runtime environment.
+type Platform struct {
+	OS   string // "linux", "windows", "darwin"
+	Arch string // "amd64", "arm64"
+
+	IsTermux  bool // Running inside Termux (ANDROID_ROOT set)
+	IsWSL     bool // Running inside WSL
+	IsDocker  bool // Running inside a Docker container
+	IsWindows bool // runtime.GOOS == "windows"
+	IsLinux   bool // runtime.GOOS == "linux"
+	IsDarwin  bool // runtime.GOOS == "darwin"
+	IsARM64   bool // runtime.GOARCH == "arm64"
+}
+
+// Detect returns the current platform information.
+func Detect() *Platform {
+	p := &Platform{
+		OS:        runtime.GOOS,
+		Arch:      runtime.GOARCH,
+		IsWindows: runtime.GOOS == "windows",
+		IsLinux:   runtime.GOOS == "linux",
+		IsDarwin:  runtime.GOOS == "darwin",
+		IsARM64:   runtime.GOARCH == "arm64",
+	}
+
+	// Detect Termux
+	if _, ok := os.LookupEnv("ANDROID_ROOT"); ok {
+		p.IsTermux = true
+	}
+
+	// Detect WSL
+	if _, err := os.Stat("/proc/sys/fs/binfmt_misc/WSL"); err == nil {
+		p.IsWSL = true
+	}
+
+	// Detect Docker
+	if _, err := os.Stat("/.dockerenv"); err == nil {
+		p.IsDocker = true
+	}
+
+	return p
+}
+
+// DataDir returns the default application data directory.
+func (p *Platform) DataDir() string {
+	home, _ := os.UserHomeDir()
+	switch {
+	case p.IsWindows:
+		return filepath.Join(os.Getenv("APPDATA"), "Cyrene")
+	case p.IsDarwin:
+		return filepath.Join(home, "Library", "Application Support", "Cyrene")
+	case p.IsTermux:
+		return filepath.Join(home, "storage", "shared", "Cyrene")
+	default: // Linux
+		return filepath.Join(home, ".local", "share", "cyrene")
+	}
+}
+
+// ConfigDir returns the default configuration directory.
+func (p *Platform) ConfigDir() string {
+	home, _ := os.UserHomeDir()
+	switch {
+	case p.IsWindows:
+		return filepath.Join(os.Getenv("APPDATA"), "Cyrene", "config")
+	case p.IsDarwin:
+		return filepath.Join(home, "Library", "Application Support", "Cyrene", "config")
+	case p.IsTermux:
+		return filepath.Join(home, ".config", "cyrene")
+	default: // Linux
+		return filepath.Join(home, ".config", "cyrene")
+	}
+}
+
+// CacheDir returns the default cache directory.
+func (p *Platform) CacheDir() string {
+	home, _ := os.UserHomeDir()
+	switch {
+	case p.IsWindows:
+		return filepath.Join(os.Getenv("LOCALAPPDATA"), "Cyrene", "cache")
+	case p.IsDarwin:
+		return filepath.Join(home, "Library", "Caches", "Cyrene")
+	default: // Linux / Termux
+		return filepath.Join(home, ".cache", "cyrene")
+	}
+}
+
+// EnsureDirs creates the data, config, and cache directories if they don't exist.
+func (p *Platform) EnsureDirs() error {
+	for _, dir := range []string{p.DataDir(), p.ConfigDir(), p.CacheDir()} {
+		if err := os.MkdirAll(dir, 0o755); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// DefaultNATSURL returns the default NATS connection URL.
+func (p *Platform) DefaultNATSURL() string {
+	return "nats://localhost:4222"
+}
+
+// DefaultPGHost returns the default PostgreSQL host.
+func (p *Platform) DefaultPGHost() string {
+	return "localhost"
+}
+
+// Summary returns a human-readable platform description.
+func (p *Platform) Summary() string {
+	s := p.OS + "/" + p.Arch
+	var tags []string
+	if p.IsDocker {
+		tags = append(tags, "docker")
+	}
+	if p.IsWSL {
+		tags = append(tags, "wsl")
+	}
+	if p.IsTermux {
+		tags = append(tags, "termux")
+	}
+	if len(tags) > 0 {
+		s += " ("
+		for i, t := range tags {
+			if i > 0 {
+				s += ", "
+			}
+			s += t
+		}
+		s += ")"
+	}
+	return s
+}