Fleet: Unauthenticated download of in-house iOS app binaries via predictable URLs

### Summary Two endpoints serving in-house iOS application packages and manifests in Fleet's enterprise tier are reachable without a hard-to-guess token in the URL, allowing an unauthenticated attacker who can reach the Fleet server to download an in-house IPA by guessing sequential title identifiers. ### Impact By design, Apple's `InstallEnterpriseApplication` MDM command requires that the manifest URL be reachable by the managed device without a Fleet session, so these endpoints cannot enforce session-based authentication. Fleet's legacy MDM installer path mitigates this by embedding a random, hard-to-guess token in the URL; the in-house iOS app endpoints (added later) were always intended to use the same time-limited-token pattern but the mitigation was not yet in place. The result is read-only disclosure of in-house IPA binaries and their metadata (bundle identifier, version, name) that an operator has deployed through Fleet. This is enterprise-tier only — the free tier returns `fleet.ErrMissingLicense`. There is no privilege escalation, write access, or impact on hosts not managed by Fleet. ### Workarounds If an immediate upgrade is not possible: - Restrict network access to the Fleet server to trusted networks, as is typical for MDM deployments. - Remove in-house iOS apps that contain sensitive material from Fleet; in-house app IPAs are intentionally reachable by managed devices and should not be relied on as a confidential distribution channel. - Where available, confi