Add get_subscription_age read function (Issue #21)

[Demilade10]

Closes #630

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Implements Issue #21.

• Added created_at: u64 to the Subscription struct
• subscribe() sets created_at on new subscriptions and resubscribes
• Added get_subscription_age(env, user) -> Option<u64>
• Bumped schema to v3; migration sets created_at = 0 as sentinel for existing subscriptions
• Added unit tests covering age calculation, resubscribe reset, no-subscription case, and migrated sentinel case

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