feat: map FastAPI projects to fastapi-patterns and api-design

project-stack-mappings.json had no `fastapi` entry, so /project-init
detected FastAPI projects as generic `python` and never resolved the
fastapi-patterns skill that already ships in the repo. api-design is
included for the same reason: FastAPI work is REST surface work.

Indicators match `fastapi` in requirements.txt or pyproject.toml,
following the existing django entry pattern. Data-only change; no
code reads this file, it is the lookup table /project-init consults.

config/project-stack-mappings.json

@@ -485,6 +485,34 @@
         "deny": []
       }
     },
+    {
+      "id": "fastapi",
+      "name": "FastAPI (Python)",
+      "indicators": [
+        { "file": "requirements.txt", "contains": "fastapi" },
+        { "file": "pyproject.toml", "contains": "fastapi" }
+      ],
+      "rules": ["common", "python"],
+      "skills": [
+        "fastapi-patterns",
+        "api-design",
+        "python-patterns",
+        "python-testing",
+        "tdd-workflow",
+        "verification-loop"
+      ],
+      "commands": {
+        "build": ["pip install -e ."],
+        "test": ["pytest", "python -m pytest"],
+        "lint": ["ruff check .", "mypy ."],
+        "format": ["ruff format .", "black ."],
+        "dev": ["uvicorn main:app --reload", "fastapi dev"]
+      },
+      "permissions": {
+        "allow": ["python *", "pip install *", "pytest *", "ruff *", "black *", "mypy *", "uvicorn *"],
+        "deny": []
+      }
+    },
     {
       "id": "android",
       "name": "Android (Kotlin/Java)",