feat(mindspace): add opt-in local analytics

docs/local-analytics.md

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+# Local Memind + Umami analytics
+
+The integration is local-only by default. Memind proxies `/analytics/*` to the
+local Umami service at `http://127.0.0.1:3100`; it does not contact 103/105.
+
+1. Start `/Users/john/Project/memind-analytics` and verify:
+
+   ```bash
+   curl --fail http://127.0.0.1:3100/api/heartbeat
+   ```
+
+2. Create one Umami Website for the local generated-page host. Do not create a
+   Website per page or per user.
+
+3. Put the Website ID and a local-only pseudonymization secret in Memind's
+   `.env`:
+
+   ```dotenv
+   MEMIND_ANALYTICS_ENABLED=true
+   MEMIND_ANALYTICS_URL=http://127.0.0.1:3100
+   MEMIND_ANALYTICS_WEBSITE_ID=<website-id>
+   MEMIND_ANALYTICS_ID_SECRET=<random-local-secret>
+   MEMIND_ANALYTICS_DOMAINS=127.0.0.1,localhost
+   ```
+
+4. Restart the local Memind server. Full generated HTML pages will receive a
+   same-origin `/analytics/script.js` tracker and a `page_view` event with
+   pseudonymous `owner_id`, `page_id`, and `channel` dimensions.
+
+The integration is fail-open: missing configuration, disabled analytics, or a
+down Umami service leaves page generation and page delivery unchanged. User
+facing analytics must be queried through a future Memind API that filters by
+the authenticated owner; do not expose the Umami dashboard directly to users.