feat: normalize JIRA signal metadata and labeling

[neethika12]

Why this change

JIRA signals were partially supported, but labels and metadata fallback behavior were inconsistent in the signal board display.

What changed

• Updated signal response typing to include jira as a supported source type
• Added optional JIRA metadata fields used by backend payloads (issueType, priority, projectKey, assignees)
• Normalized source label output to use uppercase JIRA consistently
• Added safer subtitle fallback for JIRA signals when project key metadata is missing

User impact

• JIRA signals render with clearer and more consistent labeling
• Signal rows remain readable even when some JIRA metadata is missing

Validation

• npm test -- --watch=false --browsers=ChromeHeadless (all tests passing)