feat(auth): make email the unique login identifier

Login is now by email, not username. username still exists internally
(JWT sub claim, audit logs, Jira actor attribution, SSO provisioning all
still key off it) but is now always kept equal to email everywhere a user
is created or their email changes — never a separately-chosen value.

- User.email is now unique + NOT NULL (migration b067 backfills any
  missing/blank email from username first, so existing rows — notably
  the seeded admin, which historically had none — never violate it).
- /auth/login and the (unused but updated for consistency)
  authenticate_user() now query by email.
- create_user (legacy, unreferenced but kept) and
  create_user_without_password both derive username from email.
- update_user keeps username in sync when email changes, and rejects
  duplicate emails.
- seed.py reads ADMIN_EMAIL (new env var, wired through install.sh and
  docker-compose.prod.yml) for the initial admin; falls back to an
  email-shaped ADMIN_USERNAME or a placeholder that's flagged for the
  operator to fix.
- admin_config.py's import bundle now matches/creates users by email,
  skipping (not crashing on) entries with no email.
- sso_service.py always sets username = email for SSO-provisioned users.
- LoginPage/auth.ts updated to email input/copy (wire field name stays
  'username' — that's the OAuth2PasswordRequestForm spec, not the value).

backend/app/services/sso_service.py

@@ -177,8 +177,9 @@ def process_callback(db: Session, request_data: dict) -> User:
     name_id   = auth.get_nameid()
 
     # Attribute claim URIs — defaults support both plain names and Azure AD full URIs
+    # (attr_username is no longer read: email is the sole login identifier
+    # platform-wide, username always mirrors it — see below.)
     email_attr    = cfg.attr_email    or "email"
-    username_attr = cfg.attr_username or "email"   # Azure AD: use email as username
     role_attr     = cfg.attr_role     or "http://schemas.microsoft.com/ws/2008/06/identity/claims/role"
 
     # Resolve email: try configured attr → Azure email claim URI → NameID
@@ -189,9 +190,9 @@ def process_callback(db: Session, request_data: dict) -> User:
         or ""
     )
 
-    # Resolve username: keep full email (e.g. user@company.com) to avoid collisions with local accounts
-    raw_username = _first_attr(attrs, username_attr) or email or name_id or ""
-    username = raw_username.strip() or email.split("@")[0] or name_id
+    # Email is the unique login identifier platform-wide — username always
+    # mirrors it (see User model), never a separately-provisioned IdP value.
+    username = email
 
     # Resolve role: try configured attr → Azure role claim URI → default
     role = (