feat: add Front conversation issue integration

[jaeyunha]

Summary

• add Front integration setup/disconnect plus signed sidebar endpoints for issue search/create/link/unlink
• persist Front conversation links as provider source metadata and expose linked source backlinks on issue details
• queue Front follow-up jobs to post an internal note and reopen conversations when linked issues complete or cancel
• repair integration settings Front/Zendesk setup forms after branch sync and add focused UI/unit coverage

Verification

• pnpm exec vitest run tests/integrations-view.test.tsx
• pnpm exec biome check apps/web/src/app/(app)/settings/integrations/page.tsx apps/web/tests/integrations-view.test.tsx
• make check attempted; blocked at typecheck by pre-existing missing web deps (drizzle-orm, better-auth, @better-auth/passkey)
• go test ./internal/integrations attempted; blocked because go is not installed in this runner

Notes

• Customer/customer_request mapping depends on the Parity: add Customer Requests data model, customer pages, and issue/project linking #556 data model landing; this PR stores provider source metadata without adding a parallel customer model.

[jaeyunha]

Controller disposition for current head b34a356: validation-blocked / rebase required; do not merge as-is.

Evidence:

• PR feat: add Front conversation issue integration #715 targets staging but does not contain current origin/staging; GitHub reports mergeable=CONFLICTING.
• Merge simulation conflicts across integration startup/router/catalog files, front.go, handler_test.go, issue source-link handling, and generated OpenAPI/SDK.
• Branch includes stale P1 Front: create/link issues from conversations and reopen on resolution #577 history plus the newer hardening/setup-form commits; the intended delta needs to be ported onto current staging cleanly.
• Lane report says focused Front integrations-view Vitest and Biome passed, but make check/Go validation were blocked in-lane; no GitHub checks are reported.

Required path:

• Rebase/port the Front hardening/setup-form delta onto current origin/staging, preserve already-landed Salesforce/Jira/Zendesk/Gong/Sentry source-link changes during conflict resolution, regenerate OpenAPI/SDK from current contract, then rerun focused Go + web + SDK gates before merge.