Input validation — enum checks for frequency/tone/depth/timezone

[howwohmm]

Problem

`main.py:126-134` — `EnrollRequest` accepts arbitrary strings for `frequency`, `tone`, `depth`, `timezone`:

• Bad timezone (e.g., "not_a_tz") → crash in pytz
• Bad frequency (e.g., "99x") → no email slots generated, silent failure
• `EmailStr` is imported but unused — email validation is just `"@" in email` (line 401)

Solution

• Use Pydantic `Literal` or `Enum` types for frequency, tone, depth
• Validate timezone against `pytz.all_timezones`
• Use `EmailStr` for email fields
• Add `@validator` for active_days (must be valid day abbreviations)

Files

• `main.py` — `EnrollRequest` model (lines 126-134), OTP models

Acceptance Criteria

• Invalid frequency/tone/depth returns 422 with clear error
• Invalid timezone returns 422
• Email validated properly
• Active days validated against Mon-Sun