Add COUNTER type with auto-increment and IsNegative field

COUNTER type:
- Only accepts integer values (rejects decimals)
- Auto-increment behavior: each mark adds to existing value
- Supports decrement via negative values (e.g., -2)
- Enforces minimum value of 0 (never negative)
- Default increment is 1 if no value provided

VALUE type:
- Accepts decimal values
- Replaces value (no auto-increment)

IsNegative field:
- New boolean field on Habit entity
- Persisted in database (is_negative column)
- Available in all DTOs and HTTP responses
- Marks habits as "negative" (e.g., candy consumption)

Examples:
- COUNTER: 5 + (-2) = 3, 2 + (-3) = 0, 0 + (-1) = 0
- VALUE: 5000 → 12000 = 12000 (replacement)

internal/infrastructure/persistence/sqlite/migrations.go

@@ -42,6 +42,7 @@ CREATE TABLE IF NOT EXISTS habits (
 	specific_days TEXT,
 	specific_dates TEXT,
 	carry_over BOOLEAN DEFAULT 0,
+	is_negative BOOLEAN DEFAULT 0,
 	target_value REAL,
 	created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
 	archived_at DATETIME,