fix: align comment post URL schema and imports - #28

Summary

• add the nullable post_url column to the canonical Comment schema
• make the runtime schema check add the column to existing databases when needed
• preserve post_url through comment and full-backup imports
• keep older backups without post_url compatible by importing NULL

Root cause

The comment API already reads, writes, and edits Comment.post_url, and the deployment migration can add it, but the canonical schema and runtime schema check did not guarantee that the column existed. Import paths also dropped the value during backup round trips.

Compatibility

The migration is additive and idempotent. Existing rows keep NULL, databases already containing the column are unchanged, and older backup payloads remain supported.

Validation

• ./node_modules/.bin/vitest run (5 files, 15 tests passed)
• git diff --check

tsc --noEmit still reports the six pre-existing errors covered by #26; this change introduces no additional TypeScript errors.