fix: Populate possible_values and set data_type='Enum' for enum variables

[SakshiKekre]

Summary

• Populates possible_values field with JSON-encoded values when a variable has enumerated options
• Sets data_type to "Enum" for variables with possible_values, enabling frontend dropdowns
• Fixes the issue where state_name and other enum variables rendered as text inputs instead of dropdowns

Context

The frontend's VariableInput.tsx only renders a <Select> dropdown when data_type === "Enum". Previously, enum variables had their Python type name (e.g., "str") as data_type, causing text inputs to render instead.

Test plan

• Re-seed database and verify state_name variable has data_type: "Enum" and possible_values: ["AL", "AK", ...]
• Verify frontend renders state dropdown in household builder

🤖 Generated with Claude Code

[SakshiKekre]

⚠️ Dependency: This PR depends on #92 being merged first.

Background: The app-v2-migration branch has failing tests due to direct commits that added country_id to computation module functions without updating tests. PR #92 fixes those tests.

Merge order:

1. Merge fix: Update computation module tests for country_id parameter #92 (test fix)
2. Rebase this PR on updated app-v2-migration
3. Then this PR will pass CI

[anth-volk]

@SakshiKekre Just merged #92. Could you rebase and merge?

[anth-volk]

@SakshiKekre please confirm that the app-v2-migration branch already properly serialized and deserializes into/out of this structure, e.g., when displaying the param setter. If it doesn't, please also add that handling to this PR.

[SakshiKekre]

@anth-volk Thanks for the merge.
And yes, I confirmed that serialization/deserialization works correctly:

• Serialization (API -> DB): Our fix in seed_models.py uses json.dumps() to write possible_values as JSON for PostgreSQL COPY
• Deserialization (DB -> API): Already handled by sa_column=Column(JSON) in models/variable.py - SQLAlchemy auto-converts JSONB -> Python list

[anth-volk]

@SakshiKekre Sorry, can you fully rebase all of this again? Made some changes to fix a couple bugs and improve code quality.

[SakshiKekre]

@anth-volk Thanks for the heads up. Rebased, good to go now.