fix(auth): require email authentication every login

CRITICAL SECURITY FIX:
- Email code required EVERY login (authentication, not verification)
- DNS TXT check cached separately (domain verification)
- New auth_sessions table for per-login state
- Codes hashed with SHA-256, constant-time comparison
- Max 3 attempts, 10-minute session expiry
- OAuth params stored server-side (security improvement)

New files:
- services/auth_session.py
- migrations 004, 005
- ADR-010: domain verification vs user authentication

312 tests passing, 86.21% coverage

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

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

src/gondulf/database/migrations/004_create_auth_sessions.sql

@@ -0,0 +1,35 @@
+-- Migration 004: Create auth_sessions table for per-login authentication
+--
+-- This migration separates user authentication (per-login email verification)
+-- from domain verification (one-time DNS check). See ADR-010 for details.
+--
+-- Key principle: Email code is AUTHENTICATION (every login), never cached.
+
+-- Auth sessions table for temporary per-login authentication state
+-- This table stores session data for the authorization flow
+CREATE TABLE auth_sessions (
+    session_id TEXT PRIMARY KEY,
+    me TEXT NOT NULL,
+    email TEXT,
+    verification_code_hash TEXT,
+    code_verified INTEGER NOT NULL DEFAULT 0,
+    attempts INTEGER NOT NULL DEFAULT 0,
+    client_id TEXT NOT NULL,
+    redirect_uri TEXT NOT NULL,
+    state TEXT,
+    code_challenge TEXT,
+    code_challenge_method TEXT,
+    scope TEXT,
+    response_type TEXT DEFAULT 'id',
+    created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    expires_at TIMESTAMP NOT NULL
+);
+
+-- Index for expiration-based cleanup
+CREATE INDEX idx_auth_sessions_expires ON auth_sessions(expires_at);
+
+-- Index for looking up sessions by domain (for email discovery)
+CREATE INDEX idx_auth_sessions_me ON auth_sessions(me);
+
+-- Record this migration
+INSERT INTO migrations (version, description) VALUES (4, 'Create auth_sessions table for per-login authentication - separates user authentication from domain verification');