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/models/enums.py

@@ -55,3 +55,8 @@ class CitationConfidence(enum.StrEnum):
 class AuditActorType(enum.StrEnum):
     user = "user"
     assistant = "assistant"
+
+
+class TokenPurpose(enum.StrEnum):
+    email_verify = "email_verify"
+    password_reset = "password_reset"