Store explicit side-effect severity

[Airwhale]

Stack from main: #121 -> #119 -> #122 -> #118 -> #120 -> #142 -> #141 -> #144

The first five PRs are merged. The remaining open stack is #142 -> #141 -> #144.

Why

The classifier currently stores side effects as symptom strings. That preserves which effect was reported but loses whether the author described it as mild, moderate, severe, or life-threatening. As a result, analyses can compare side-effect reporting rates but cannot test whether dose or route is associated with reported severity.

The extraction must remain conservative. A symptom that sounds clinically serious is not evidence that the author experienced it severely, so severity should remain missing unless the author explicitly states it.

Approach taken

• Add a small Pydantic SideEffectReport model with side_effect and nullable severity fields.
• Restrict severity to mild, moderate, severe, and life_threatening.
• Update batch and fallback prompts to return side-effect objects and explicitly prohibit inferring severity from the symptom name.
• Store the objects as JSON in the existing treatment_reports.side_effects text column. No schema migration or rewrite of existing rows is required.
• Add focused tests for valid severity, invalid severity, unspecified severity, and the exact JSON written to SQLite.

User-facing changes

New classification rows store side effects as objects such as:

{"side_effect": "headache", "severity": "mild"}

When the author reports a side effect without describing its intensity, severity is null. Existing database rows are unchanged and retain their original string-array JSON.

Detailed test plan

1. Run the complete test suite:

uv run pytest -q

Expected result: 379 passed.

2. Run the focused contract and persistence tests:

uv run pytest tests/test_side_effect_severity.py -q

Expected result: 3 passed.

3. Run lint checks on the new model, prompt, persistence code, and tests:

uvx ruff check src/models.py src/prompts/intervention_config.py src/utilities/db.py tests/test_side_effect_severity.py

Expected result: All checks passed!

4. Verify the stacked diff contains only the intended code, schema comment, documentation, and tests:

git diff --name-only origin/codex/match-dose-administration-route...HEAD

Expected files:

README.md
schema.sql
src/models.py
src/pipeline/classify.py
src/prompts/intervention_config.py
src/utilities/db.py
tests/test_side_effect_severity.py