feat: bounty search email subscriptions (#1141)

[FhiPunkVn]

Summary

Implements #1141 — email when a new bounty matches saved search criteria.

What

• BountyEmailSubscription model: query tokens, min_amount, tracker_name, language
• API CRUD: /api/user/bounty_email_subscriptions
• Mailer bounty_search_match (text + html) with unsubscribe category
• On bounty_placed, async notify matching active subscriptions
• Spec for matches? criteria

Notes

• Reuses existing bounty_alerts unsubscribe pattern
• Does not email the bounty poster
• Payment / contact: USDC Base 0xfd3600efdfcd6f02c515f93237e2caaff293769f if bounty escrow still active

Test plan

• Run migration CreateBountyEmailSubscriptions
• Create subscription via API with query + min_amount
• Place a matching bounty → email queued
• Non-matching bounty → no email
• Unsubscribe category works

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