feat: add config-driven AI model tier routing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/tests/test_config_model_tiers.py

@@ -0,0 +1,24 @@
+"""Tests for AI model tier configuration."""
+from app.core.config import settings
+
+
+def test_ai_model_tiers_exist():
+    assert "fast" in settings.AI_MODEL_TIERS
+    assert "standard" in settings.AI_MODEL_TIERS
+
+
+def test_action_model_map_covers_all_actions():
+    valid_tiers = set(settings.AI_MODEL_TIERS.keys())
+    for action, tier in settings.ACTION_MODEL_MAP.items():
+        assert tier in valid_tiers, f"Action '{action}' maps to unknown tier '{tier}'"
+
+
+def test_get_model_for_action():
+    model = settings.get_model_for_action("generate_full")
+    assert isinstance(model, str)
+    assert len(model) > 0
+
+
+def test_get_model_for_action_unknown_falls_back():
+    model = settings.get_model_for_action("nonexistent_action")
+    assert model == settings.AI_MODEL_TIERS["standard"]