Migrate email signups (courses + drip delivery) from Brevo to Loops

Decision direction: use **loops.so** for email signups and course drip delivery, replacing Brevo (hitting automation/delivery limits). Newsletter stays on Substack.

## Scope
1. Loops account setup; verify sending domain.
2. Recreate the 3 course sequences as Loops loops (source markdown already in git: developmentalspaces.org repo `learn/courses/` and devspaces-consolidation `sources/brevo/markdown/` — 30 emails incl. images).
3. New signup forms/endpoints per course; embed or link on /learn and each course page (replacing the 4 legacy sibforms.com URLs, which stay live until cutover).
4. Add practice-psychology course when ready.
5. Migrate existing Brevo course-list contacts (export from Brevo; mind consent/segments).
6. Decommission Brevo automations after cutover; keep archive export.

## Notes
- Courses are now web-canonical (readable at /learn/courses/…); email = optional paced experience, so drip logic can be simple (signup → day 0..N, one/day).
- Manifesto-signing automation (#34) should target the same Loops list — coordinate.
- Blocked on: Loops account/API key from Rufus. After that, most steps are automatable via Loops API.

Start with learn/courses/ and sources/brevo/markdown/ to inventory the 30 emails and images, then inspect /learn and each course page for the four legacy sibforms.com links. Confirm the Loops account/API key and coordinate the shared list with #34 before implementing signup endpoints, sequences, contact migration, and cutover. Done means the three courses deliver one email per day, consent and segments are preserved, forms are live, and Brevo is decommissioned with an archive.

Written by the indexing model from the issue text.