feat(mail-evidence): import read-only Gmail evidence

[max5800]

Summary

• Add a real read-only Gmail payload converter for mail evidence imports
• Add POST /api/v1/mail-evidence/import while keeping /mock-import for compatibility
• Add a gws-based Gmail import script with dry-run and quality gating
• Improve German invoice amount/payment parsing and avoid false refund detection from shop boilerplate
• Document the Gmail mail-evidence import workflow and safety boundary

Verification

• uv run pytest tests/test_mail_evidence_layer.py -> 17 passed
• uv run ruff check src/api/routers/mail_evidence.py src/external/gmail_import.py src/services/mail_evidence.py scripts/import_gmail_evidence.py tests/test_mail_evidence_layer.py
• uv run alembic heads -> 0027_mail_evidence_analysis_context (head)
• OpenAPI check confirms /api/v1/mail-evidence/import
• uv run pytest -> 369 passed, 317 skipped
• Gmail dry-run against real mailbox: 10 seen, 2 finance evidence candidates, 8 skipped by quality gate, 0 Gmail writes

[github-actions]

🎉 This PR is included in version 1.42.0 🎉

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