feat(#76): IMP-47B reject-as-AI-adaptation activation (u1~u13 backend + tests)

- u1~u9: AI fallback infrastructure (router/prompts/schema/validator) + Step 12 hook
- u10: e2e reject chain (writes final.html with AI-repaired slot, full coverage)
- u11: frontend wiring deferred to follow-up commit (split from IMP-41 hunks)
- u12: coverage_invariant guard
- u13: cache save gate (visual_check PASS + user_approved/auto_cache) — Codex #22 verified

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/config.py

@@ -26,6 +26,14 @@ class Settings(BaseSettings):
     ai_fallback_budget_per_run: int = 10
     ai_fallback_circuit_breaker_threshold: int = 5
 
+    # IMP-46 u5 — auto-cache flag. When True, `save_proposal` bypasses the
+    # `user_approved` gate only (`visual_check_passed` is never bypassed).
+    # Default OFF preserves the dual-gate contract; the CLI flag
+    # `--auto-cache` in `src/phase_z2_pipeline.py` mutates this setting at
+    # parse time. Downstream callers MUST source the flag from Settings,
+    # never inline literals.
+    ai_fallback_auto_cache: bool = False
+
     model_config = {"env_file": ".env", "env_file_encoding": "utf-8"}