PATCH /api/integrations/:id merges arbitrary config keys — needs an allowlist

[lilyshen0722]

From sprint-review's #1290 pass: { ...currentConfig, ...config } (routes/integrations.ts) accepts any config key from any caller who passes canDeleteIntegration — including botToken, secretToken, accessToken, globalAgentAccess, agentAccessEnabled. Pre-existing on main; #1290 added a targeted guard for linkedUserId only (the live-relay attribution identity). The general fix is a per-type allowlist of client-settable keys, with credential-class and privilege-class keys excluded by default.

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