Add repetition_date support for tasks (v3.3.0)

- Add repetition_date field to tasks table (migration 018)
- Support pattern-based repetition: day of week, day of month, specific date
- Add 'Через'/'Каждое' mode selector in task form
- Auto-calculate next_show_at from repetition_date on create/complete
- Show calculated next date in postpone dialog for repetition_date tasks
- Update version to 3.3.0

play-life-backend/migrations/018_add_repetition_date.sql

@@ -0,0 +1,16 @@
+-- Migration: Add repetition_date field to tasks table
+-- This script adds the repetition_date field for pattern-based recurring tasks
+-- Format examples: "2 week" (2nd day of week), "15 month" (15th day of month), "02-01 year" (Feb 1st)
+
+-- ============================================
+-- Add repetition_date column
+-- ============================================
+ALTER TABLE tasks 
+ADD COLUMN IF NOT EXISTS repetition_date TEXT;
+
+-- ============================================
+-- Comments for documentation
+-- ============================================
+COMMENT ON COLUMN tasks.repetition_date IS 'Pattern-based repetition: "N week" (day of week 1-7), "N month" (day of month 1-31), "MM-DD year" (specific date). Mutually exclusive with repetition_period.';
+
+