fix(api): keep anonymous submitter handles private

[dvd233]

What & why

is_anonymous controls whether a submission's author identity is public, but the create and edit routes still persisted a supplied handle when it was set to true. The public post mapper also returned handles from legacy anonymous rows, so a direct API consumer could recover an identity the submitter intended to hide.

Closes #102

Changes

• Drop submitter_handle on create and edit whenever is_anonymous is true.
• Hide legacy handles in the public post mapper while preserving handles for attributed posts.
• Add regression coverage for create, edit, legacy public reads, and the non-anonymous path.

Testing

• pnpm exec vitest run — 145 tests across 16 files passed.
• pnpm run lint — passed with no warnings or errors.
• pnpm exec tsc --noEmit — passed.
• Prettier check and git diff --check — passed.
• pnpm run build was attempted with local placeholder environment values. Compilation and type checking passed, but Next.js 14.2.35 failed while prerendering the existing /icon and /apple-icon routes on native Windows with TypeError: Invalid URL from next/og. No icon or build configuration files are part of this change.

[github-actions]

Merged. Thanks a lot, @dvd233.

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