[Feature]: E2E happy-path coverage

Parent PRD

#242

What to build

Update the existing Subscribe-via-marketplace E2E happy path so it walks the rewritten Subscription workflow end-to-end and asserts the full set of side effects.

End-to-end behavior:

• Playwright (or whatever E2E harness the existing subscribe test uses) drives a consumer through the rewritten Subscription workflow.
• The test asserts: an agreement record is created, a PDF is stored at the configured volume path, an in-app notification is delivered to the signer, and (when configured) an email is dispatched.

See "Testing Decisions" → "End-to-End" in the parent PRD.

Acceptance criteria

• E2E test walks legal_document → acknowledgement_checklist → co_signers → persist_agreement → generate_pdf → deliver
• Test asserts exactly one agreement created
• Test asserts pdf_storage_path is set on the agreement
• Test asserts the signer receives an in-app notification
• Test runs in CI without flake on at least 3 consecutive green runs

Blocked by

• Blocked by #252

User stories addressed

• User story 10
• User story 17