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>

.env.example

@@ -0,0 +1,36 @@
+# =============================================================================
+# Tanabata File Manager — environment variables
+# Copy to .env and fill in the values.
+# =============================================================================
+
+# ---------------------------------------------------------------------------
+# Server
+# ---------------------------------------------------------------------------
+LISTEN_ADDR=:8080
+JWT_SECRET=change-me-to-a-random-32-byte-secret
+JWT_ACCESS_TTL=15m
+JWT_REFRESH_TTL=720h
+
+# ---------------------------------------------------------------------------
+# Database
+# ---------------------------------------------------------------------------
+DATABASE_URL=postgres://tanabata:password@localhost:5432/tanabata?sslmode=disable
+
+# ---------------------------------------------------------------------------
+# Storage
+# ---------------------------------------------------------------------------
+FILES_PATH=/data/files
+THUMBS_CACHE_PATH=/data/thumbs
+
+# ---------------------------------------------------------------------------
+# Thumbnails
+# ---------------------------------------------------------------------------
+THUMB_WIDTH=160
+THUMB_HEIGHT=160
+PREVIEW_WIDTH=1920
+PREVIEW_HEIGHT=1080
+
+# ---------------------------------------------------------------------------
+# Import
+# ---------------------------------------------------------------------------
+IMPORT_PATH=/data/import