fix: create file storage directory on initialization

[Mabolla]

Summary

• create the configured file storage directory during NewFileStorage initialization
• add a regression test covering a missing storage directory

Why

The file backend initializes backfill_processes and lockfile immediately, but previously assumed the configured directory already existed. On a fresh deployment with a new --file-directory, those metadata writes can fail because the parent directory is missing.

Closes #71.

Validation

• reproduced the failure locally with a regression test
• go test ./common/storage -v
• git diff --cached --check

S3 integration tests were skipped because RUN_INTEGRATION_TESTS was not set.

[cb-heimdall]

🟡 Heimdall Review Status

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[Mabolla]

Closing this PR because #72 was opened earlier and addresses the same issue (#71). Thanks.