Notion connector

[kiranfin]

Short summary

Accept NOTION as a source system in the AI service, so chunks ingested from the Notion connector keep their source system and can be filtered in chat.

• Added NOTION to the SourceSystem literal and to is_source_system in src/rag/types.py, so Notion chunks are no longer rejected or dropped during ingestion.
• Added NOTION to SourceSystemValue in src/api/schemas.py, so the chat request filter accepts it.
• Added tests for the ingest round trip, the case-insensitive normalization and the chat filter (tests/rag/test_filters.py, tests/api/test_chat.py).

Checks

• I verified the code makes sense intuitively
• The PR changes affect only this issue, no unrelated/unwanted code changes to other modules/code segments
• CI runs (./gradlew clean build, keycloack)
• New business logic is unit tested, including WebMvcTest for api controllers
• The new functionality is tested manually