Add server time endpoint for authoritative Unix timestamp checks

[rob1997]

What

Clients need an authoritative current Unix timestamp from the indexer/API boundary to avoid relying on local device time when evaluating subscription expiry.

How to Reproduce

1. Evaluate Subscription.endTime on the client.
2. Compare behavior across clients with skewed local clocks.
3. Observe inconsistent access decisions when local time is inaccurate.

Expectation

Provide a lightweight endpoint that returns current server Unix time for consistent subscription validity checks.

Additional Context (optional)

There is already work in #24 for server-side active subscription checks. This issue should either complement that endpoint (if still needed) or be resolved by documenting why #24 is sufficient.

Estimation

Dependencies

• Related to feat(api): add GET /subscriptions/active endpoint for server-side expiry check #24 (server-side expiry handling).

[dhyaniarun1993]

@rob1997 Is this required?