Add configurable welcome flow with group rules

[SaharPak]

Context

Currently the bot sends a hardcoded welcome DM after approving a join request. This should be configurable and could include group rules.

Requirements

• Move welcome message template to config (or a templates/ folder)
• Support variables: {first_name}, {username}, {group_name}
• Optionally include a "rules acknowledgement" button (inline keyboard)
• Support multiple languages (English + Farsi at minimum)

Acceptance Criteria

• Welcome message is configurable without code changes
• Supports at least EN and FA
• Inline button for rules acknowledgement (optional feature flag)