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>

lib/db/client.ts

@@ -7,10 +7,13 @@ const globalForDb = globalThis as unknown as {
   _client: ReturnType<typeof createClient> | undefined;
 };
 
+const DATA_DIR = process.env.DATA_DIR || "./data";
+const DATABASE_URL = process.env.DATABASE_URL || `file:${DATA_DIR}/sqlite.db`;
+
 function getClient() {
   if (!globalForDb._client) {
     globalForDb._client = createClient({
-      url: process.env.DATABASE_URL ?? "file:./data/sqlite.db",
+      url: DATABASE_URL,
     });
     globalForDb._client.execute("PRAGMA journal_mode = WAL");
     globalForDb._client.execute("PRAGMA foreign_keys = ON");