Add timestamp validation for incoming Nostr events

[j0KZ]

Summary

• Validates event timestamps to reject clearly invalid events and mitigate replay attacks
• Different event types have different tolerance windows based on their nature

Validation Rules

Event Type	Kind	Window	Rationale
Ephemeral events	20000-29999	±1 hour	Should be recent, used for presence/chat
Gift wraps (NIP-17)	1059	±24 hours	NIP-17 uses randomized timestamps for privacy
Other events	*	Future ≤5 min	Historical events OK, reject clearly invalid futures

Changes

• Add hasValidTimestamp() method to NostrEvent struct
• Validate timestamp in ParsedInbound alongside signature validation
• Document the validation rules with clear comments

Test plan

• Verify ephemeral events outside ±1 hour are rejected
• Verify gift wraps within ±24 hours are accepted
• Verify events >5 min in the future are rejected
• Verify historical events (e.g., old text notes) are accepted
• Verify normal operation of location channels and private messages

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