Bitbucket changes

[kiranfin]

Short summary

Adds BITBUCKET as a known source system in the AI service so Bitbucket content can be ingested, tagged and filtered like GitHub, Jira and Confluence.

• SourceSystem and is_source_system in src/rag/types.py now include BITBUCKET. Before this, ingested Bitbucket chunks silently ended up without a source system, so a Bitbucket filter could never match them.
• SourceSystemValue in src/api/schemas.py now accepts BITBUCKET, so the chat endpoint no longer rejects a Bitbucket source filter with a 422.
• New tests check that a Bitbucket chunk keeps its source system through ingest and filtering, that the chat endpoint accepts BITBUCKET/bitbucket as a filter and answers from Bitbucket chunks, that unknown systems like GITLAB are still rejected, and that source system names are matched case insensitively.

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 (gitleaks, ruff check, ruff format --check, pyright, pytest)
• New business logic is unit tested, including API tests for the FastAPI endpoints
• The new functionality is tested manually