[codex] Add Apollon feature model entry

[JTNing]

Summary

• Added Apollon as an optional Platform Integrations feature in functional-feature-model.json.
• Added a guided workflow option for Apollon and included it in integration review/template references.
• Kept changes limited to JSON resources under src/main/resources/feature-model.

Artemis Evidence

• src/main/java/de/tum/cit/aet/artemis/core/config/Constants.java: MODULE_FEATURE_APOLLON and APOLLON_ENABLED_PROPERTY_NAME = "artemis.apollon.enabled".
• src/main/resources/config/application-core.yml: artemis.apollon.enabled defaults to false and documents the Apollon conversion service requirement.
• src/main/java/de/tum/cit/aet/artemis/modeling/config/ApollonEnabled.java: Spring condition for Apollon-controlled beans.
• src/main/java/de/tum/cit/aet/artemis/modeling/config/ModelingWithApollonEnabled.java: confirms Apollon is tied to Modeling workflows.
• src/main/java/de/tum/cit/aet/artemis/modeling/web/ApollonConversionResource.java: exposes UML model PDF conversion when Apollon is enabled.
• src/main/webapp/app/modeling/manage/detail/modeling-exercise-detail.component.ts: checks MODULE_FEATURE_APOLLON for modeling exercise details.

Candidate Decisions

• Added: Apollon. It is teacher-visible, functionally separable, controlled by explicit Artemis module/config anchors, and comparable to existing optional integration features such as EduIDE and Sharing Platform.
• Not added: LDAP, SAML2, and passkey module switches. They are authentication/security infrastructure rather than teacher-facing functional model choices at the current granularity.
• Not added: Weaviate/global search. It has explicit technical anchors, but the evidence points to a search backend/runtime feature toggle rather than a stable course/module-level functional feature for this model.
• Not added: TumLive. It is an optional video-source condition, but it is narrower than the current coarse feature model groups and needs stronger metadata before adding.

Verification

• jq empty src/main/resources/feature-model/*.json
• ./gradlew test --rerun-tasks
• npm run test
• npm run build

Known Uncertainties

• Artemis does not currently contain .artemis-feature-model/ source-controlled metadata, so this update relies on source/config evidence only.

[JTNing]

Correction to the previous follow-up comment: the shell stripped inline-code formatting from that comment.

Follow-up update for Apollon semantics:

• Added apollon-requires-modeling as a requires constraint (apollon -> modeling) because Apollon PDF conversion is only meaningful for modeling exercise UML models in this functional model.
• Updated the guided configurator to hide options that would select a feature while leaving its requires constraints unsatisfied. This hides Apollon unless Modeling is selected.
• Added cleanup so if Modeling is later deselected, an already-selected Apollon guided option is removed from the selection instead of leaving a hidden invalid choice.
• Updated backend/API tests for the non-empty constraints array and added frontend coverage for the Apollon visibility/cleanup behavior.

Verification after the follow-up:

• jq empty src/main/resources/feature-model/*.json
• ./gradlew test --rerun-tasks
• npm run test
• npm run build