Add canonical source matching with alias overrides

## Parent PRD

#1

## What to build

Introduce the new cross-source identity rule by matching stalls on normalized name plus mappable place signal plus country, with alias overrides as a safety valve. This slice should prove that source unification can happen without relying on the old cuisine-based identity heuristic.

## Acceptance criteria

- [ ] Cross-source matching uses normalized name plus mappable place signal plus country as the primary identity rule.
- [ ] Alias overrides can resolve naming drift without forcing bad automatic merges.
- [ ] Tests cover matching behavior, alias behavior, and the preference for false negatives over false positives.

## Blocked by

- Blocked by #6

## User stories addressed

Reference by number from the parent PRD:

- User story 16
- User story 22
- User story 23
- User story 24
- User story 25
- User story 27
- User story 30