feat: implement admin login

Implement Story 1.2 (Admin Login) with full TDD approach including:

- RateLimit model for tracking authentication attempts
- LoginForm for admin authentication with email, password, and remember_me fields
- Rate limiting utility functions (check, increment, reset)
- admin_required decorator for route protection
- Login route with rate limiting (5 attempts per 15 minutes)
- Logout route with session clearing
- Admin dashboard now requires authentication
- Login template with flash message support
- 14 comprehensive integration tests covering all acceptance criteria
- Email normalization to lowercase
- Session persistence with configurable duration (7 or 30 days)

All acceptance criteria met:
- Login form accepts email and password
- Invalid credentials show appropriate error message
- Successful login redirects to admin dashboard
- Session persists across browser refreshes
- Rate limiting after 5 failed attempts

Test coverage: 90.67% (exceeds 80% requirement)
All linting and type checking passes

Story: 1.2

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/forms/login.py

@@ -0,0 +1,39 @@
+"""Login form for admin authentication."""
+
+from flask_wtf import FlaskForm
+from wtforms import BooleanField, EmailField, PasswordField
+from wtforms.validators import DataRequired, Email
+
+
+class LoginForm(FlaskForm):
+    """Form for admin login.
+
+    Validates email format and requires password.
+
+    Attributes:
+        email: Email address for admin account.
+        password: Password for admin account.
+        remember_me: Whether to extend session duration.
+    """
+
+    email = EmailField(
+        "Email Address",
+        validators=[
+            DataRequired(message="Email address is required."),
+            Email(message="Please enter a valid email address."),
+        ],
+        render_kw={"placeholder": "admin@example.com"},
+    )
+
+    password = PasswordField(
+        "Password",
+        validators=[
+            DataRequired(message="Password is required."),
+        ],
+        render_kw={"placeholder": "Enter your password"},
+    )
+
+    remember_me = BooleanField(
+        "Remember me",
+        default=False,
+    )