[18.0][ADD] sign_oca_reminder: reminders, resend, and expiration

[manana2520]

Summary

Adds DocuSign-like reminder, resend, and expiration functionality to sign_oca.

Features

• Manual Resend: "Resend" button on sent sign requests to re-notify unsigned signers
• Automatic Reminders: Daily cron sends periodic reminders at a configurable interval (default: 3 days)
• Request Expiration: Optional validity date; requests past this date are automatically cancelled by the cron
• Company Defaults: Configure default reminder settings in Settings > Sign OCA
• Search Filters: "Expiring Soon" and "Expired" filters in the sign request list view
• Post-init hook: Backfills sent_date for existing sent requests so reminders work immediately after install

Technical Details

• Extends sign.oca.request with new fields: sent_date, reminder_enabled, reminder_interval_days, last_reminder_date, reminder_count, validity_date, next_reminder_date
• Extends sign.oca.request.log with new actions: resend, expire, reminder
• Extends res.company and res.config.settings for company-level defaults
• Single daily cron handles both expiration and reminder logic
• Depends only on sign_oca (no other dependencies)

Test Coverage

23 test cases covering:

• Reminder sending and interval computation
• Expiration and automatic cancellation
• Cron behavior for both reminders and expiration
• Computed fields (next_reminder_date, is_expired)
• Edge cases (no signers, already signed, disabled reminders)
• Manual resend functionality
• Default values from company settings
• Post-init hook backfill

Screenshots

The module adds reminder/expiration fields to the sign request form:

(Will add screenshots if requested by reviewers)

Migration Notes

• No migration scripts needed - this is a new module
• Post-init hook safely handles existing databases with sent requests