fix: guard feature backfill catalog access

[pjy008008]

What changed

• construct the feature-backfill operating catalog through PostgresCatalog.connect
• explicitly select StorageObjectsPolicy.READ_ONLY for the planner
• add a regression test that requires the guarded factory and read-only policy

Why

The deployed backfill_features entry point still constructed PostgresCatalog with the old signature. After storage ownership became explicit, the read-only planning command failed before reading the development catalog:

TypeError: PostgresCatalog.__init__() missing 1 required keyword-only argument: 'storage_objects'

Using the guarded factory also keeps the operator planner from receiving storage-object write authority it does not need.

Impact

The §2.5 operator can once again inspect the four-resolution RSI backfill plan before sending any SQS commands. No schema, migration, or feature calculation behavior changes.

Validation

• regression test failed before the fix and passes after it
• pytest tests/test_feature_backfill.py -q: 38 passed
• non-integration suite excluding the existing Windows long-path test: 1047 passed, 1 skipped, 72 subtests passed
• mypy apps/pipeline_worker/backfill_features.py: passed
• Ruff on changed files: passed