feat: smart payee & merchant alias management (#114)

[alexchenai]

Summary

Implements smart payee normalization and custom alias management for clean merchant names.

Features

• Auto-normalization: 20+ built-in rules (AMZN*MKTP US -> Amazon, NETFLIX.COM -> Netflix)
• Strips common prefixes (TST*, SQ*, PP*) and suffixes (LLC, Inc, reference numbers)
• Custom alias CRUD: override any canonical name with a personal alias
• Bulk normalization API for processing entire transaction lists
• Alias suggestions: auto-surfaces normalization candidates for user confirmation
• Per-user alias storage for personalization

API Endpoints

• GET /api/payees/aliases — list all custom aliases
• POST /api/payees/aliases — create/update alias {raw_name, alias}
• DELETE /api/payees/aliases/{canonical} — remove alias
• POST /api/payees/normalize — bulk normalize {raw_names: [...]}
• POST /api/payees/suggestions — get alias suggestions {raw_names: [...]}

Tests

9 tests: normalization rules, custom CRUD, bulk normalize, suggestions.

Closes #114