refactor(hash): dedupe edit-token hashing into shared helper

[waterlemonnn]

Closes #31.

Edit tokens were hashed with an inline createHash("sha256").update(token).digest("hex") in six separate places (three local hashToken/getTokenHash helpers, three fully inline). Added hashEditToken() to lib/utils/hash.ts, alongside the existing hashIp/getClientIp, and pointed all six call sites at it:

• app/api/posts/route.ts
• app/api/status/[token]/route.ts
• app/api/edit/[token]/route.ts
• app/api/posts/edit/[token]/route.ts
• app/api/admin/posts/[id]/resend-token/route.ts
• app/edit/[token]/page.tsx

Algorithm is unchanged — same sha256(token) hex digest as before, so existing posts.edit_token_hash rows still resolve. randomBytes token generation in route.ts/resend-token/route.ts was left alone per the issue.

Added a test in lib/utils/hash.test.ts (determinism + a fixed known digest) copying the shape of the existing hashIp/getClientIp tests.

app/api/posts/route.test.ts mocks @/lib/utils/hash with an explicit object instead of importOriginal, so it needed a hashEditToken mock added too — otherwise the route's new import throws inside the mocked module.

Verify

npx tsc --noEmit   # clean
npm run lint       # clean
npm test           # 117/117 passing