phase A: user accounts + session-cookie auth

Replaces the static bearer-token gate with a real auth boundary. The
existing CASSANDRA_TOKEN path is retained as an admin / scripting escape
hatch — kept compatible by aliasing require_token to require_auth.

- New users table (migration 0007): email, argon2 password_hash, tier,
  email_verified (declared but not enforced until phase E), settings_json
  for the tone/analysis/anchor knobs we'll wire in phase D.
- app/services/auth_service.py: argon2-cffi password hashing with timing-
  attack-resistant authenticate() (always runs a hash verify even on
  unknown-email to deny a username-enumeration oracle).
- app/auth.py rewritten: require_auth returns a CurrentUser with either
  is_admin=True (bearer path) or a User object (session path). Failing
  requests get 303 → /login for HTML, 401 for API. Sessions signed with
  itsdangerous against CASSANDRA_SESSION_SECRET; 14-day TTL.
- app/routers/auth.py: /login, /signup, /logout. Login form preserves the
  ?next=… param for redirect-after-login. Signup respects a new
  CASSANDRA_SIGNUP_ENABLED flag.
- Standalone /login + /signup templates (no app chrome). base.html grows
  a user chip + logout link in the header (reads request.state.current_user).

Phase A's main known limitations are documented in the plan: email
verification is declared but not enforced; session revocation is
best-effort (cookie-only, not DB-backed). Both land in phase E.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

alembic/versions/0007_users.py

@@ -0,0 +1,40 @@
+"""users table — accounts, password hashing (argon2), tier, settings
+
+Phase A of the multi-user migration. Adds the table but doesn't add owner
+FKs to existing rows yet — that's phase C. Until then, data is still
+effectively shared across the (small) set of authenticated accounts.
+
+Revision ID: 0007
+Revises: 0006
+Create Date: 2026-05-16
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+
+revision: str = "0007"
+down_revision: Union[str, None] = "0006"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "users",
+        sa.Column("id", sa.Integer, primary_key=True, autoincrement=True),
+        sa.Column("email", sa.String(255), nullable=False),
+        sa.Column("password_hash", sa.String(255), nullable=False),
+        sa.Column("tier", sa.String(16), nullable=False, server_default="free"),
+        # Not enforced in phase A — wired up in phase E.
+        sa.Column("email_verified", sa.Boolean, nullable=False, server_default=sa.text("0")),
+        sa.Column("settings_json", sa.JSON),   # per-user tone/analysis/anchor/...
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("last_login_at", sa.DateTime(timezone=True)),
+        sa.UniqueConstraint("email", name="uq_users_email"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("users")