Add consented social milestone posting

docs/automations/social-milestone-worker.md

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+# Social milestone worker
+
+The app detects QR-code scan milestones and stores customer consent. It does not
+hold X or LinkedIn credentials. An external X worker can use the internal queue
+after the test rollout is approved.
+
+## Test setup (manual SQL only)
+
+1. Apply [`sql/2026-08-13_social_milestones.sql`](../../sql/2026-08-13_social_milestones.sql)
+   to `qrmaster_test`.
+2. Set distinct `CRON_SECRET` and `INTERNAL_API_SECRET` values in `.env.test`.
+   For an end-to-end test without 1,000 scans, also set
+   `SOCIAL_MILESTONE_THRESHOLDS=1` (or `1,2`). Do not set this on production.
+   Publishing is immediate after consent by default. Set
+   `SOCIAL_MILESTONE_POST_DELAY_HOURS=24` only if a revocation window is desired.
+3. Deploy using the documented test compose command. `CRON_SECRET` is forwarded
+   to the web service by `docker-compose.yml`.
+4. Trigger detection manually:
+
+```bash
+curl -H "Authorization: Bearer $CRON_SECRET" \
+  https://testmodul.qrmaster.net/api/cron/social-milestones
+```
+
+The detector creates records at 1,000 and 10,000 unique scans only. It is safe
+to call repeatedly because `(qrId, kind)` is unique.
+
+## X worker contract
+
+After an explicit rollout approval, the existing QRMaster X worker may poll:
+
+```bash
+curl -H "Authorization: Bearer $INTERNAL_API_SECRET" \
+  https://qrmaster.net/api/internal/social-milestones
+```
+
+It receives at most one approved item per 24 hours. The worker must post `milestone.text`
+without altering it, then report its result:
+
+```bash
+curl -X PATCH -H "Authorization: Bearer $INTERNAL_API_SECRET" \
+  -H "Content-Type: application/json" \
+  -d '{"id":"<milestone-id>","result":"posted"}' \
+  https://qrmaster.net/api/internal/social-milestones
+```
+
+Do not configure this worker against `testmodul`. LinkedIn has no approved
+brand-posting integration in this project. Customers can self-share: X opens a
+prefilled intent; the same text is copied for a LinkedIn post.