feat(academy): add course enrollment and gate task submissions

[V1ctor-o]

Summary

• Add course enrollment and withdrawal endpoints with capacity checks.
• Require completion of prerequisite courses before enrollment.
• Enforce enrollment states: active, withdrawn, and completed.
• Reject course task submissions unless the learner has an active enrollment.
• Add unit and endpoint tests for these rules.

Testing

• Course service tests: passed.
• Course endpoint test: passed.
• Full backend tests and type check remain blocked by existing errors in the chat and health modules.

Course and enrollment data currently use in-memory storage, consistent with the existing BackendAcademy service pattern; they do not survive a restart.

Closes #793

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[MaryammAli]

LGTM