[Backend] -- Improve Referral Attribution Consistency (Timing, Replay, Dedup)

[Devsol-01]

Description:
Referral attribution depends on event timing (signup, deposit, withdrawal). If events are replayed or delayed, attribution logic must remain consistent.

Implement:

• deduplication for attribution events
• deterministic attribution windows
• idempotency for referral crediting

Acceptance Criteria:

• Referral credits computed deterministically.
• Replay safe.
• Tests validate delayed events.

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