Monthly pricing re-review

[github-actions]

Pricing tags can go stale without the link ever breaking, so this is a
monthly spot-check of a rotating slice of the list.

This batch is 15 of 185 entries (the sample rotates each month and covers the whole list about every 13 months).

For each entry, open the link and confirm the pricing tag still matches:

• (free) — still free, no paywall on the useful parts
• (freemium) — still has a genuinely usable free tier
• (paid) — still paid

Tick a box once confirmed. If one is wrong, open a PR fixing the tag (and
note it under CHANGELOG.md's Unreleased if you remove the entry).

• Student Minds — (free) — Mental Health & Wellbeing
• The Trevor Project — (free) — Mental Health & Wellbeing
• Academia Stack Exchange — (free) — Communities
• AnitaB.org — (free) — Communities
• ESN (Erasmus Student Network) — (free) — Communities
• Focusmate — (freemium) — Communities
• I'm First — (free) — Communities
• Mathematics Stack Exchange — (free) — Communities
• oSTEM — (free) — Communities
• r/APStudents — (free) — Communities
• r/GetStudying — (free) — Communities
• r/HomeworkHelp — (free) — Communities
• r/IBO — (free) — Communities
• r/igcse — (free) — Communities
• r/SAT — (free) — Communities

See the Quality Standards and the pricing
guidance in CONTRIBUTING.md.