feat: integrate shared AI provider with safe execution evidence

[jameswilson87156-del]

Summary

• adds project-first and shared PORTFOLIO_AI_* provider configuration
• enforces explicit chat-completions protocol compatibility
• adds controlled provider error classification and sanitization
• persists requested, actual, and fallback execution metadata
• adds exact safe HTTP failure observability
• adds Flyway V5 and V6 migrations
• isolates all automated tests from host provider configuration
• records offline and synthetic real-provider validation evidence

Validation

• Maven tests: 42 passed
• Maven package: passed
• synthetic real-provider validation: passed
• final real request: HTTP 200
• requested and actual provider/model metadata matched
• fallback was disabled and unused
• Generation Record, Trace, Agent Run, and Agent Step persisted
• secret scan passed
• frontend, README, screenshots unchanged

Claims Boundary

This PR validates one synthetic Chat Completions path. It does not claim production readiness, long-term provider stability, SLA guarantees, broad model compatibility, or complete real-world failure coverage.