Add Supabase target provider

[MaxGhenis]

Summary

• add SupabaseTargetProvider to convert Supabase target rows into core TargetSpec/TargetSet objects
• preserve raw SupabaseTargetLoader behavior while adding provider/query filters for period, entity, target type, states, institution, and unsupported-variable inclusion
• classify target metadata including source, stratum, target type, US category/level, and whether the variable is mapped to a known column

Verification

• UV_FROZEN=1 uv run --extra dev ruff check src/microplex_us/supabase_targets.py tests/targets/test_supabase.py
• UV_FROZEN=1 uv run --extra dev pytest -q tests/targets/test_supabase.py tests/test_package_imports.py
• UV_FROZEN=1 uv run --extra dev --with pydantic --with-editable ../microplex pytest -q tests/test_package_imports.py tests/targets/test_supabase.py tests/pipelines/test_check_site_snapshot.py tests/pipelines/test_imputation_ablation.py tests/pipelines/test_site_snapshot.py tests/pipelines/test_version_benchmark.py
• GitNexus impact: low risk, 0 affected processes
• GitNexus detect-changes: low risk, 0 affected processes

Note: tests/test_target_adapters.py in the plain dev env still requires optional h5py; this PR does not change that dependency boundary.