Close all 25 codex audit findings and add KMP contract tests

Remediate all P0-S priority findings from cross-platform architecture audit:
- Add input validation and authorization checks across handlers
- Harden social auth (Apple/Google) token validation
- Add document ownership verification and file type validation
- Add rate limiting config and CORS origin restrictions
- Add subscription tier enforcement in handlers
- Add OpenAPI 3.0.3 spec (81 schemas, 104 operations)
- Add URL-level contract test (KMP API routes match spec paths)
- Add model-level contract test (65 schemas, 464 fields validated)
- Add CI workflow for backend tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/backend-ci.yml

@@ -0,0 +1,75 @@
+name: Backend CI
+
+on:
+  push:
+    branches: [main, develop]
+  pull_request:
+    branches: [main, develop]
+
+jobs:
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+          cache: true
+
+      - name: Download dependencies
+        run: go mod download
+
+      - name: Run tests
+        run: go test -race -count=1 ./...
+
+      - name: Run contract validation
+        run: go test -v -run "TestRouteSpecContract|TestKMPSpecContract" ./internal/integration/
+
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    needs: test
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+          cache: true
+
+      - name: Build API
+        run: go build -ldflags "-s -w" -o bin/casera-api ./cmd/api
+
+      - name: Build Worker
+        run: go build -ldflags "-s -w" -o bin/casera-worker ./cmd/worker
+
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+          cache: true
+
+      - name: Run go vet
+        run: go vet ./...
+
+      - name: Check formatting
+        run: |
+          unformatted=$(gofmt -l .)
+          if [ -n "$unformatted" ]; then
+            echo "Unformatted files:"
+            echo "$unformatted"
+            exit 1
+          fi