Add auth foundation: sessions/tokens schema, Argon2 hashing, config

Two tables (sessions, user_tokens) + migration; only token *hashes* are stored, so a DB leak yields no usable credential. Argon2id password hashing and token primitives in app/core/security. Config and .env.example gain session/cookie/token TTLs, app base URL, and SMTP settings (twelve-factor). Migration verified reversible (drops the token_purpose enum) and matches the models.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/core/security.py

@@ -0,0 +1,35 @@
+"""Password hashing and token primitives.
+
+Passwords use Argon2id (argon2-cffi). Session and email tokens are random
+high-entropy strings; only their SHA-256 hash is stored, so a database leak
+never exposes a usable credential.
+"""
+
+import hashlib
+import secrets
+
+from argon2 import PasswordHasher
+from argon2.exceptions import Argon2Error
+
+_hasher = PasswordHasher()
+
+
+def hash_password(password: str) -> str:
+    return _hasher.hash(password)
+
+
+def verify_password(password_hash: str, password: str) -> bool:
+    try:
+        return _hasher.verify(password_hash, password)
+    except (Argon2Error, ValueError):
+        return False
+
+
+def generate_token() -> str:
+    """A URL-safe, high-entropy token (the raw secret handed to the client)."""
+    return secrets.token_urlsafe(32)
+
+
+def hash_token(token: str) -> str:
+    """SHA-256 of a token — what we store and look up by."""
+    return hashlib.sha256(token.encode("utf-8")).hexdigest()