Fix silent deletes: 204 handling, batch-delete guard, mutation feedback

[CaptainCo0L]

Problem

Deleting an item (e.g. a batch) gave no visible result — the page didn't refresh and there was no success/error indication.

Root causes

1. api.js parsed empty 204 bodies as JSON. DELETE returns 204 No Content, but request() always called res.json(), throwing "Unexpected end of JSON input". That thrown error skipped the list reload(), so the deleted item stayed on screen. → Return null on 204.
2. delete_batch 500'd on dependents. Raw db.delete + commit; Batch has no cascade and the FKs (BatchEnrollment/Session/Payment → batch_id) have no ondelete, so deleting a batch with students/sessions/payments raised a Postgres FK error. → Block with a clean 409 naming the blockers (active enrollments / sessions / payments), and sweep the dead soft-deleted join rows left by unenroll before deleting. No destructive cascade.
3. No feedback on any mutation button. → Wrapped delete / remove / unenroll / deactivate / disable handlers in try/catch with success/error toasts (the toast system already existed and was used by other pages).

Files

• frontend/src/api.js — 204 → null
• backend/app/routers/batches.py — 409 guard + dead-row cleanup
• frontend/src/pages/{Batches,Tutors,Users,StudentDetail}.jsx — toast feedback

Verification (Docker + Playwright)

• 🔴 Delete batch with a student → red "Can't delete: batch still has 1 enrolled student(s)…" toast, card stays, HTTP 409 (not 500)
• 🟢 Remove student → "Student removed." toast
• 🟢 Delete now-empty batch → "Batch deleted." toast, card disappears, HTTP 204
• Create flows unregressed (8/8)

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