fix: route gtm through telemetry entrypoint (#8354)

Wire checkout attribution into GTM events and checkout POST payloads.

This updates the cloud telemetry flow so the backend team can correlate checkout events without relying on frontend cookie parsing. We now surface GA4 identity via a GTM-provided global and include attribution on both `begin_checkout` telemetry and the checkout POST body. The backend should continue to derive the Firebase UID from the auth header; the checkout POST body does not include a user ID.

GTM events pushed (unchanged list, updated payloads):
- `page_view` (page title/location/referrer as before)
- `sign_up` / `login`
- `begin_checkout` now includes:
  - `user_id`, `tier`, `cycle`, `checkout_type`, `previous_tier` (if change flow)
  - `ga_client_id`, `ga_session_id`, `ga_session_number`
  - `gclid`, `gbraid`, `wbraid`

Backend-facing change:
- `POST /customers/cloud-subscription-checkout/:tier` now includes a JSON body with attribution fields only:
  - `ga_client_id`, `ga_session_id`, `ga_session_number`
  - `gclid`, `gbraid`, `wbraid`
- Backend should continue to derive the Firebase UID from the auth header.

Required GTM setup:
- Provide `window.__ga_identity__` via a GTM Custom HTML tag (after GA4/Google tag) with `{ client_id, session_id, session_number }`. The frontend reads this to populate the GA fields.

<img width="1416" height="1230" alt="image" src="https://github.com/user-attachments/assets/b77cf0ed-be69-4497-a540-86e5beb7bfac" />

## Screenshots (if applicable)

<img width="991" height="385" alt="image" src="https://github.com/user-attachments/assets/8309cd9e-5ab5-4fba-addb-2d101aaae7e9"/>

Manual Testing:
<img width="3839" height="2020" alt="image" src="https://github.com/user-attachments/assets/36901dfd-08db-4c07-97b8-a71e6783c72f"/>
<img width="2141" height="851" alt="image" src="https://github.com/user-attachments/assets/2e9f7aa4-4716-40f7-b147-1c74b0ce8067"/>
<img width="2298" height="982" alt="image" src="https://github.com/user-attachments/assets/72cbaa53-9b92-458a-8539-c987cf753b02"/>
<img width="2125" height="999" alt="image" src="https://github.com/user-attachments/assets/4b22387e-8027-4f50-be49-a410282a1adc"/>

To manually test, you will need to override api/features in devtools to also return this:

```
"gtm_container_id": "GTM-NP9JM6K7"
```

┆Issue is synchronized with this [Notion page](https://www.notion.so/PR-8354-fix-route-gtm-through-telemetry-entrypoint-2f66d73d36508138afacdeffe835f28a) by [Unito](https://www.unito.io)


<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

* **New Features**
  * Analytics expanded: page view tracking, richer auth telemetry (includes user IDs), and checkout begin events with attribution.
  * Google Tag Manager support and persistent checkout attribution (GA/client/session IDs, gclid/gbraid/wbraid).

* **Chores**
  * Telemetry reworked to support multiple providers via a registry with cloud-only initialization.
  * Workflow module refactored for clearer exports.

* **Tests**
  * Added/updated tests for attribution, telemetry, and subscription flows.

* **CI**
  * New check prevents telemetry from leaking into distribution artifacts.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.github/workflows/ci-dist-telemetry-scan.yaml

@@ -0,0 +1,52 @@
+name: 'CI: Dist Telemetry Scan'
+
+on:
+  pull_request:
+    branches-ignore: [wip/*, draft/*, temp/*]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  scan:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+
+      - name: Install pnpm
+        uses: pnpm/action-setup@41ff72655975bd51cab0327fa583b6e92b6d3061 # v4.2.0
+        with:
+          version: 10
+
+      - name: Use Node.js
+        uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
+        with:
+          node-version: 'lts/*'
+          cache: 'pnpm'
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Build project
+        run: pnpm build
+
+      - name: Scan dist for telemetry references
+        run: |
+          set -euo pipefail
+          if rg --no-ignore -n \
+            -g '*.html' \
+            -g '*.js' \
+            -e 'Google Tag Manager' \
+            -e '(?i)\bgtm\.js\b' \
+            -e '(?i)googletagmanager\.com/gtm\.js\\?id=' \
+            -e '(?i)googletagmanager\.com/ns\.html\\?id=' \
+            dist; then
+            echo 'Telemetry references found in dist assets.'
+            exit 1
+          fi
+          echo 'No telemetry references found in dist assets.'