chore: harden GitHub collaboration and security automation

.github/CODEOWNERS

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+* @mkarson1997
+
+/.github/ @mkarson1997
+/src/ @mkarson1997
+/migrations/ @mkarson1997
+/tests/ @mkarson1997

.github/PULL_REQUEST_TEMPLATE.md

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+## Summary
+
+Describe the workflow-runtime change and why it is needed.
+
+## Reliability impact
+
+- [ ] Retry / lease / idempotency behavior was considered where relevant.
+- [ ] Database and migration compatibility was considered where relevant.
+- [ ] Observability or failure-mode impact was considered where relevant.
+
+## Validation
+
+- [ ] `npm run typecheck`
+- [ ] `npm test`
+- [ ] `npm run build`
+- [ ] No credentials, tokens, production data, or connection strings were committed.
+
+## Notes
+
+Add migration notes, operational trade-offs, benchmark evidence, or follow-up work when useful.

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: npm
+    directory: /
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 5
+    groups:
+      npm-development-dependencies:
+        dependency-type: development
+
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 5

.github/workflows/codeql.yml

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+name: CodeQL
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+  schedule:
+    - cron: "21 4 * * 3"
+
+permissions:
+  contents: read
+  security-events: write
+
+jobs:
+  analyze:
+    name: Analyze JavaScript/TypeScript
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@v3
+        with:
+          languages: javascript-typescript
+
+      - name: Perform CodeQL analysis
+        uses: github/codeql-action/analyze@v3