FAC-145 feat: faculty composite overall rating (50/25/25)

[y4nder]

Summary

Adds a faculty-level Overall Rating that aggregates the three questionnaire tracks with a 50 / 25 / 25 weighting:

• 50% Faculty Feedback
• 25% Out-of-Classroom
• 25% In-Classroom

Handles six coverage states (FULL / PARTIAL / PARTIAL_NO_FEEDBACK / FEEDBACK_ONLY / INSUFFICIENT / NO_DATA) including the Dean-respecting null-on-missing-FEEDBACK behaviour.

Scope

Backend

• New endpoint: GET /api/v1/analytics/faculty/:facultyId/overview?semesterId&courseId?
• Reuses per-type rating code path from the faculty report for parity
• Chain-of-rounding invariant: composite.rating === round2(Σ non-null contribution[i])

Frontend

• CompositeRatingSummaryStrip (labelled "Overall rating") + breakdown popover above tabs
• Moved per-type rating strip into Scores and Feedback tabs, directly under the questionnaire-type sub-tabs (resolves a redundancy with the composite at top)
• Renamed user-facing "Composite rating" label → "Overall rating"

Tech spec

See _bmad-output/implementation-artifacts/tech-spec-faculty-composite-overall-rating.md.