Enable Dependabot for dependency security scanning

Enables automated PRs for dependency updates and security
vulnerabilities. Will catch issues like Mako CVE-2026-44307.

References: #1812
Change-Id: I852513232af7251cd2056843f1d9c7b5635efaab

Author: zzzeek

.github/dependabot.yml

@@ -0,0 +1,36 @@
+# Dependabot configuration for Alembic
+# Automatically creates PRs for dependency updates and security vulnerabilities
+
+version: 2
+updates:
+  # Python dependencies
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    # Create PRs for both security updates and version updates
+    open-pull-requests-limit: 10
+    labels:
+      - "dependencies"
+      - "python"
+    # Group all patch updates together to reduce noise
+    groups:
+      patch-updates:
+        patterns:
+          - "*"
+        update-types:
+          - "patch"
+    # Allow both minor and major version updates
+    allow:
+      - dependency-type: "direct"
+      - dependency-type: "indirect"
+
+  # GitHub Actions dependencies
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    labels:
+      - "dependencies"
+      - "github-actions"