refactor(auth): remove obsolete CONVERT_PASSWORDS env var

Legacy md5/argon2id hashes are now always upgraded to bcrypt on login, so
the CONVERT_PASSWORDS flag is no longer used. Drop it from env schema,
.env.example, the docker installer, and test mocks.

src/actions/auth-precheck.test.ts

@@ -17,7 +17,7 @@ const core = vi.hoisted(() => ({
 	}),
 }));
 
-vi.mock("@/env", () => ({ env: { CONVERT_PASSWORDS: false } }));
+vi.mock("@/env", () => ({ env: {} }));
 vi.mock("@/lib/auth/login-core", () => core);
 vi.mock("@/lib/rate-limit", () => ({ clientIp: vi.fn(), rateLimit: vi.fn() }));
 vi.mock("@/lib/services/captcha", () => ({