[18.0][ADD] sign_oca_order: sequential signing and CC recipients

[manana2520]

Summary

Adds sequential (ordered) signing and CC recipient functionality to OCA Sign, similar to DocuSign's routing and carbon copy features.

Note: This PR depends on #150 (sign_oca_reminder) being merged first, as sign_oca_order extends reminder functionality for sequential-aware reminder behavior.

Features

• Sequential Signing Mode: Signers are ordered by a signing_order field. Only the current step's signers are notified; others wait.
• Parallel Groups: Signers with the same order number sign simultaneously within that step.
• CC Recipients: Partners who receive an email with the signed PDF attached after all signers complete.
• Signer Status Tracking: Each signer progresses through draft -> waiting -> sent -> signed states.
• Portal "Waiting" Page: Signers who aren't yet current in the sequence see a "please wait" page instead of the signing form.
• Sequential-Aware Reminders: Reminders only go to the current signing step's unsigned signers (not future steps).
• Signed Copy Distribution: After all signers complete, each signer receives a copy of the signed document.

Technical Details

• Extends sign.oca.request with: signing_mode (parallel/sequential), cc_partner_ids, current_signing_order
• Extends sign.oca.request.signer with: signing_order, signer_state (draft/waiting/sent/signed)
• Extends sign.oca.request.log with new actions: advance_step, cc_notify
• Overrides portal controller to show "waiting" page for non-current signers
• Overrides _cron_send_reminders for sequential awareness
• Depends on sign_oca and sign_oca_reminder

Test Coverage

42 test cases across 3 test files:

• test_sequential_signing.py: Step advancement, notification flow, signer state transitions, edge cases
• test_parallel_mode.py: Default mode behavior, all-at-once sending, CC with parallel
• test_cc_recipients.py: CC notification after completion, multiple CC recipients, no CC case

Signing Flow

Sequential Mode:
  Order 1: Alice, Bob  ->  Both notified simultaneously
  Order 2: Carol       ->  Notified only after Alice AND Bob sign
  Order 3: Dave        ->  Notified only after Carol signs
  CC: Eve, Frank       ->  Receive signed PDF after Dave signs

Parallel Mode (default):
  All signers notified at once (existing behavior preserved)

Migration Notes

• No migration scripts needed - this is a new module
• Default signing mode is "parallel", preserving existing behavior for all current requests