feat(auth): auto-create admin user profiles with highest tier and complimentary flag

- Add `is_complimentary` column to UserProfile model (migration 019)
- Update `_ensure_user_profile` to accept `is_admin` param; admins get
  highest subscription tier, is_complimentary=True, onboarding skipped
- Call `_ensure_user_profile` from all login paths (OAuth, local user, admin creds)
- Add `is_complimentary` to UserProfileUpsert schema, response helpers,
  list_users, get_user, upsert_user_profile in admin API
- Add complimentary toggle to admin users UI with gift badge in table
- Write 18 new tests covering complimentary plan and admin auto-creation
- Update SubscriptionTiers.md documentation

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

migrations/versions/019_add_is_complimentary.py

@@ -0,0 +1,30 @@
+"""Add is_complimentary column to user_profiles
+
+Revision ID: 019_add_is_complimentary
+Revises: 018_add_local_users_and_billing
+Create Date: 2026-03-07
+
+"""
+
+from typing import Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "019_add_is_complimentary"
+down_revision: Union[str, None] = "018_add_local_users_and_billing"
+depends_on: Union[str, None] = None
+
+
+def upgrade() -> None:
+    """Add is_complimentary column to user_profiles."""
+    op.add_column(
+        "user_profiles",
+        sa.Column("is_complimentary", sa.Boolean(), nullable=False, server_default="0"),
+    )
+
+
+def downgrade() -> None:
+    """Remove is_complimentary column from user_profiles."""
+    op.drop_column("user_profiles", "is_complimentary")