M1.5 backend: admin role + DB-backed settings, DB-URL-only install

- users.is_admin; first registered user becomes admin; registration gated by the
  allow_registration setting (first account always allowed). is_admin in
  /api/auth/* responses; require_admin guard (live DB check).
- settings table + code registry (site_name, allow_registration, session_ttl_days)
  with typed defaults — empty table = all defaults (rule 26). get/set/validate
  service; GET /api/config (public) + GET/PATCH /api/settings (admin), live
  session-TTL apply with no restart (rule 25).
- Cookie-signing secret now persisted in the DB (before_serving load-or-create),
  so sessions survive restarts with no volume. Config: DATABASE_URL is the only
  required env; SECRET_KEY + DATA_DIR are optional break-glass items.
- Migration 0003; DB-free tests for settings validation + admin guard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

alembic/versions/0003_roles_settings.py

@@ -0,0 +1,32 @@
+"""roles + settings
+
+Revision ID: 0003
+Revises: 0002
+Create Date: 2026-07-19
+
+Adds the admin role bit and the DB-backed settings table. No rows are seeded —
+defaults live in the code registry (thoughtsync.settings), so an empty table
+means every setting is at its default.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0003"
+down_revision = "0002"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("users", sa.Column("is_admin", sa.Boolean(), nullable=False, server_default=sa.false()))
+    op.create_table(
+        "settings",
+        sa.Column("key", sa.Text(), primary_key=True),
+        sa.Column("value", sa.Text(), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), nullable=False, server_default=sa.func.now()),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("settings")
+    op.drop_column("users", "is_admin")