feat: config, migrations embed, and server entrypoint

- internal/config: typed Config struct loaded from env vars via godotenv;
  all fields from docs (listen addr, JWT, DB, storage, thumbs, import)
- migrations/embed.go: embed FS so goose SQL files are baked into the binary
- cmd/server/main.go: load config → connect pgxpool → goose migrations
  (embedded) → Gin server with GET /health returning 200 OK
- .env.example: documents all required and optional env vars
- go.mod: bump to Go 1.26, add gin/pgx/goose/godotenv as direct deps

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/config/config.go

@@ -0,0 +1,107 @@
+package config
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strconv"
+	"time"
+
+	"github.com/joho/godotenv"
+)
+
+// Config holds all application configuration loaded from environment variables.
+type Config struct {
+	// Server
+	ListenAddr    string
+	JWTSecret     string
+	JWTAccessTTL  time.Duration
+	JWTRefreshTTL time.Duration
+
+	// Database
+	DatabaseURL string
+
+	// Storage
+	FilesPath       string
+	ThumbsCachePath string
+
+	// Thumbnails
+	ThumbWidth    int
+	ThumbHeight   int
+	PreviewWidth  int
+	PreviewHeight int
+
+	// Import
+	ImportPath string
+}
+
+// Load reads a .env file (if present) then loads all configuration from
+// environment variables. Returns an error listing every missing or invalid var.
+func Load() (*Config, error) {
+	// Non-fatal: .env may not exist in production.
+	_ = godotenv.Load()
+
+	var errs []error
+
+	requireStr := func(key string) string {
+		v := os.Getenv(key)
+		if v == "" {
+			errs = append(errs, fmt.Errorf("%s is required", key))
+		}
+		return v
+	}
+
+	defaultStr := func(key, def string) string {
+		if v := os.Getenv(key); v != "" {
+			return v
+		}
+		return def
+	}
+
+	parseDuration := func(key, def string) time.Duration {
+		raw := defaultStr(key, def)
+		d, err := time.ParseDuration(raw)
+		if err != nil {
+			errs = append(errs, fmt.Errorf("%s: invalid duration %q: %w", key, raw, err))
+			return 0
+		}
+		return d
+	}
+
+	parseInt := func(key string, def int) int {
+		raw := os.Getenv(key)
+		if raw == "" {
+			return def
+		}
+		n, err := strconv.Atoi(raw)
+		if err != nil {
+			errs = append(errs, fmt.Errorf("%s: invalid integer %q: %w", key, raw, err))
+			return def
+		}
+		return n
+	}
+
+	cfg := &Config{
+		ListenAddr:    defaultStr("LISTEN_ADDR", ":8080"),
+		JWTSecret:     requireStr("JWT_SECRET"),
+		JWTAccessTTL:  parseDuration("JWT_ACCESS_TTL", "15m"),
+		JWTRefreshTTL: parseDuration("JWT_REFRESH_TTL", "720h"),
+
+		DatabaseURL: requireStr("DATABASE_URL"),
+
+		FilesPath:       requireStr("FILES_PATH"),
+		ThumbsCachePath: requireStr("THUMBS_CACHE_PATH"),
+
+		ThumbWidth:    parseInt("THUMB_WIDTH", 160),
+		ThumbHeight:   parseInt("THUMB_HEIGHT", 160),
+		PreviewWidth:  parseInt("PREVIEW_WIDTH", 1920),
+		PreviewHeight: parseInt("PREVIEW_HEIGHT", 1080),
+
+		ImportPath: requireStr("IMPORT_PATH"),
+	}
+
+	if len(errs) > 0 {
+		return nil, errors.Join(errs...)
+	}
+	return cfg, nil
+}