fix(webhooks): ignore unhandled event types

[pttydou]

Fixes #124.

handleEvent unconditionally marked every verified event in its non-pull_request branch as PROCESSED, even though only issues ran a handler there. This change makes issues the second handled branch and marks all other verified event types IGNORED without setting processedAt. Existing pull_request, issues, and invalid-signature behavior is preserved.

Tests:

• node node_modules/jest/bin/jest.js src/github/github-webhooks.service.spec.ts --runInBand (12 passed)
• node node_modules/jest/bin/jest.js src/github --runInBand (34 passed)
• node node_modules/jest/bin/jest.js --runInBand --testPathIgnorePatterns=integration (166 passed)
• node node_modules/eslint/bin/eslint.js "{src,apps,libs,test}/**/*.ts" --fix (0 errors; 22 pre-existing e2e warnings)
• node node_modules/@nestjs/cli/bin/nest.js build

I couldn't run the seven Postgres integration tests locally because this environment has no Docker/Postgres. The remaining 169 tests passed in the full Jest run; CI provides Postgres 16.

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