## Why
Speaker identity usage is currently tracked as a denormalized transcript count. That loses which meeting files caused the count and prevents Meeting Assistant from updating past transcript files when an identity is later named or merged.

## What Changes
- Store speaker identity meeting participation as reference rows containing meeting note and transcript file addresses.
- Calculate identity usage counts from references when ordering matching candidates.
- Add transcript audit lines to referenced transcripts when an identity is named or two identities are merged.

## Impact
- Speaker identity SQLite schema gains a references table.
- Matching, attendee canonicalization, and merge ordering use calculated reference counts instead of a persisted counter.
- Existing databases keep working; legacy transcript-count columns may remain unused.