security(integrations): harden credential lifecycle boundaries

[darwin67]

Summary

• reject raw OAuth installation through API-key connect and replacement operations
• bound malformed top-level and nested credential payloads without exposing secret input in diagnostics
• close provider-specific Phoenix and LiveView parameter-filtering gaps
• normalize user-deletion races and align key-version validation with database limits
• deterministically pause in-flight encryption to test competing writes, disconnect fencing, and deletion fencing

Verification

• focused credential and lifecycle tests: 24 passed
• mix precommit: 360 passed, 5 live-provider tests excluded
• valid production key-ring runtime configuration check passed
• final Oracle review found no remaining issues in the implemented Phase 1A foundation scope

Stack

Depends on #30, which depends on #29 and #28.