feat(auth): implement unified email accounts for OAuth and local users

[kaumudpa]

Title

feat(auth): add unified email accounts - optional account linking across providers

---

What kind of change does this PR introduce?

Feature - Adds opt-in unified email accounts that allow users to access their account through any authentication provider (OAuth or email/password) using the same email address.

Why was this change needed?

Currently, Postiz treats each provider + email combination as a separate account. This creates friction for users:

1. User registered with email/password, later tries Google OAuth: Gets redirected to sign-up page instead of logging in, even though the email is the same
2. Self-hosted instances with registration disabled: OAuth users with matching emails cannot access their existing accounts
3. User flexibility: Users may want to use OAuth on mobile but password on shared computers

This PR introduces a new UNIFIED_EMAIL_ACCOUNTS environment variable (default: false) that when enabled:

• Allows OAuth login to find and use existing email/password accounts
• Allows email/password registration to add a password to existing OAuth accounts (with email verification for security)
• Enables forgot password for any account with a password set

Security: When an OAuth user adds a password, email verification is required to prevent account hijacking.

Backward compatibility: When the flag is disabled (default), all existing behavior is 100% preserved.

Fixes #1120

Other information:

Files changed:

• auth.service.ts - Core logic with flag checks
• auth.controller.ts - Simplified to use service response
• users.repository.ts - Added getUserByEmailAnyProvider(), setPassword(), setPasswordHash()
• users.service.ts - Exposed new repository methods
• .env.example - Documented new variable

New environment variable:

UNIFIED_EMAIL_ACCOUNTS=true  # Default: false (disabled)

This feature is useful for self-hosted instances that want a single account per email across multiple auth providers.

Checklist:

Put a "X" in the boxes below to indicate you have followed the checklist;

• I have read the CONTRIBUTING guide.
• I checked that there were not similar issues or PRs already open for this.
• This PR fixes just ONE issue (do not include multiple issues or types of change in the same PR) For example, don't try and fix a UI issue and include new dependencies in the same PR.

[vercel]

@kaumudpa is attempting to deploy a commit to the Listinai Team on Vercel.

A member of the Team first needs to authorize it.