Extract shared constants into lib/constants.ts

- Add `lib/constants.ts` exporting `DATA_DIR`, `DATABASE_URL`, `CACHE_DIR`,
  `BACKUP_DIR`, `AVATAR_DIR`, `TMDB_API_BASE_URL`, and `TMDB_IMAGE_BASE_URL`
- Replace inline `process.env` derivations in `db/client`, `backup`,
  `image-cache`, `system-health`, `tmdb/client`, `actions/settings`,
  and `api/avatars` with imports from the new module

lib/db/client.ts

@@ -1,7 +1,7 @@
 import { Database } from "bun:sqlite";
-import path from "node:path";
 import type { Logger } from "drizzle-orm";
 import { drizzle } from "drizzle-orm/bun-sqlite";
+import { DATABASE_URL } from "@/lib/constants";
 import { createLogger } from "@/lib/logger";
 import * as schema from "./schema";
 
@@ -29,10 +29,6 @@ const globalForDb = globalThis as unknown as {
   _client: Database | undefined;
 };
 
-const DATABASE_URL =
-  process.env.DATABASE_URL ||
-  path.join(process.env.DATA_DIR || "./data", "sqlite.db");
-
 function getClient() {
   if (!globalForDb._client) {
     globalForDb._client = new Database(DATABASE_URL);