feat: implement exchange creation

Add Exchange model, API endpoint, and form validation
for creating new gift exchanges.

- Create ExchangeForm with timezone validation
- Add admin routes for creating and viewing exchanges
- Generate unique 12-char slug for each exchange
- Validate registration/exchange dates
- Display exchanges in dashboard
- All tests passing with 92% coverage

Story: 2.1

pyproject.toml

@@ -91,3 +91,15 @@ warn_return_any = true
 warn_unused_configs = true
 disallow_untyped_defs = false
 ignore_missing_imports = true
+
+[dependency-groups]
+dev = [
+    "mypy>=1.19.1",
+    "pre-commit>=4.5.1",
+    "pytest>=9.0.2",
+    "pytest-cov>=7.0.0",
+    "pytest-flask>=1.3.0",
+    "ruff>=0.14.10",
+    "types-flask>=1.1.6",
+    "types-pytz>=2025.2.0.20251108",
+]