[Enhancement] Add priority levels to todos

[niat-web]

Feature Request

What:
Allow users to set a priority level for each todo:

• 🔴 High Priority
• 🟡 Medium Priority
• 🟢 Low Priority

Expected behavior:
Creating/editing todo:

• Priority dropdown selector
• Default: Medium Priority

Todo card:

• Color coded priority badge visible on each card
• High = red badge
• Medium = yellow badge
• Low = green badge

Sorting:

• Sort todos by priority option
• High priority todos shown first by default

Database changes needed:
ALTER TABLE todos ADD COLUMN priority VARCHAR(10) DEFAULT 'medium';

Acceptance Criteria:

• Priority selector in create todo form
• Priority selector in edit todo form
• Priority badge shown on todo card
• Todos sorted by priority by default
• Database column added