Introduce DATA_DIR env var for consistent data path configuration

Replace separate DATABASE_URL and IMAGE_CACHE_DIR env vars with a
single DATA_DIR root (default: ./data, Docker: /data). DATABASE_URL
and CACHE_DIR are derived from it but can still be overridden
individually.

Also:
- Pin pnpm to @10 for reproducible Docker builds
- Add --link to COPY instructions for better BuildKit cache reuse
- Add syntax directive for BuildKit features
- Simplify Dockerfile mkdir to just /data (ensureImageDirs handles subdirs)
- Remove redundant DATABASE_URL from docker-compose.yml

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

.env.example

@@ -1,10 +1,10 @@
-# ─── Database ───────────────────────────────────────────────────────────
-# SQLite database URL (Docker: file:/data/sqlite.db, local dev: file:sqlite.db)
-# DATABASE_URL=file:/data/sqlite.db
+# ─── Data Directory ──────────────────────────────────────────────────────
+# Root directory for SQLite database and image cache (default: ./data, Docker: /data)
+# DATA_DIR=./data
 
 # ─── TMDB (required) ───────────────────────────────────────────────────────
 # API Read Access Token — get one at https://www.themoviedb.org/settings/api
-TMDB_API_READ_ACCESS_TOKEN=your_tmdb_api_read_access_token_here
+TMDB_API_READ_ACCESS_TOKEN=
 
 # Optional: override TMDB base URLs (advanced)
 # TMDB_API_BASE_URL=https://api.themoviedb.org/3
@@ -13,7 +13,7 @@ TMDB_API_READ_ACCESS_TOKEN=your_tmdb_api_read_access_token_here
 # ─── Auth (required) ───────────────────────────────────────────────────────
 # Random secret for session encryption (min 32 chars)
 # Generate one with `npx @better-auth/cli secret` or `openssl rand -base64 32`
-BETTER_AUTH_SECRET=your_secret_here
+BETTER_AUTH_SECRET=
 # Public URL of your instance, especially important if reverse proxy is used
 BETTER_AUTH_URL=http://localhost:3000
 
@@ -28,7 +28,5 @@ BETTER_AUTH_URL=http://localhost:3000
 # DISABLE_PASSWORD_LOGIN=false  # Set to "true" to hide email/password form when OIDC is configured
 
 # ─── Image Caching ─────────────────────────────────────────────────────
-# Downloads TMDB images to local disk for faster serving (default: /data/images)
-# IMAGE_CACHE_DIR=/data/images
 # Set IMAGE_CACHE_ENABLED to "false" to use TMDB CDN directly (default: enabled)
 # IMAGE_CACHE_ENABLED=true