Add persistent settings system with database backend and comprehensive UI

- New Setting ORM model (key-value store with category, value_type, audit fields)
- Alembic migration to create the settings table
- Settings API endpoints: GET/PUT /api/v1/settings/{key}, GET /api/v1/settings (list+filter), POST /api/v1/settings/bulk
- Default seeding (17 sensible defaults across general/dmarc/dns/cloudflare/notifications categories)
- Secret redaction for cloudflare.api_token and notifications.smtp_password
- Updated settings.html: General, DMARC Policy Defaults, DNS Resolver, Cloudflare Integration, Email Notifications sections
- All forms wired to the API via Alpine.js with flash feedback
- 12 new tests for the settings model and endpoints

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/19dbc6cd-07cb-406e-b3b6-411f7721f737

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

backend/alembic/versions/add_settings_table.py

@@ -0,0 +1,38 @@
+"""add settings table
+
+Revision ID: c3d4e5f6a7b8
+Revises: b2c3d4e5f6a7
+Create Date: 2026-03-30 07:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "c3d4e5f6a7b8"
+down_revision: Union[str, Sequence[str], None] = "b2c3d4e5f6a7"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Create the settings table."""
+    op.create_table(
+        "settings",
+        sa.Column("key", sa.String(100), nullable=False),
+        sa.Column("value", sa.Text(), nullable=True),
+        sa.Column("description", sa.String(255), nullable=True),
+        sa.Column("value_type", sa.String(20), nullable=False, server_default="string"),
+        sa.Column("category", sa.String(50), nullable=False, server_default="general"),
+        sa.Column("updated_at", sa.DateTime(), nullable=True),
+        sa.Column("updated_by", sa.Integer(), nullable=True),
+        sa.ForeignKeyConstraint(["updated_by"], ["users.id"]),
+        sa.PrimaryKeyConstraint("key"),
+    )
+
+
+def downgrade() -> None:
+    """Drop the settings table."""
+    op.drop_table("settings")