fix(webhooks): isolate per-issue errors and deduplicate linked issues in handlePullRequest (#47)

[ghzhost]

Summary

Addresses Issue #47 by hardening GithubWebhooksService.handlePullRequest:

1. Deduplicates issue numbers: PR descriptions referencing an issue multiple times (e.g. "Fixes N+1 query and unbounded loop patterns across bounty expiry, milestone resolution, and reputation computation will not scale past a few hundred rows #12. Also resolves N+1 query and unbounded loop patterns across bounty expiry, milestone resolution, and reputation computation will not scale past a few hundred rows #12") are deduplicated before iteration to prevent attempting duplicate state transitions and spurious webhook failures.
2. Per-issue error isolation: Wraps each linked issue processing in a dedicated try/catch block so that a failure on one bounty does not abort processing of subsequent linked bounties in the same PR.
3. Aggregated error reporting: Collects any per-issue failures and reports them in the aggregate WebhookEvent.error field for clear operator visibility while allowing partial successes to complete.

Verification

• Added test verifying deduplication of multiple issue references in a single PR.
• Added test verifying error isolation across multiple linked issues (e.g., N+1 query and unbounded loop patterns across bounty expiry, milestone resolution, and reputation computation will not scale past a few hundred rows #12 fails while fix: global exception filter to stop leaking internal error detail to clients #34 and The entire auth module (GitHub/JWT strategies, guards, controller, service) has zero test coverage, leaving MergeFi's only identity boundary completely unverified #56 succeed).
• All unit tests pass.

Closes #47

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