schemas + rate-limit + feedback helpers + tests

- src/lib/server/schemas.ts: feedback Zod schemas (Question discriminated union + FormDefinition + Instance create/update + Submission/Post/Hide + SignIn).
- src/lib/server/rate-limit.ts (+ test): in-memory token bucket — direct port from flexsiebels.
- src/lib/server/feedback.ts: generateSlug (32-char base62), getInstanceBySlug/ById via fdb(), RATE_LIMIT constants, clampUserAgent.
- src/lib/server/public-scope.test.ts: gate behaviour tests (allowlist coverage + 6 evaluatePolicy cases). Adapted for fdbck's allowlist (no /api/share, no /api/gotify-public).
- @types/bun added so svelte-check resolves bun:test imports — clean baseline (no 'Cannot find bun:test' tech debt that the flexsiebels project carries).

bun run check: 0 errors, 0 warnings.
bun run test: 20/20 pass.

package.json

@@ -15,6 +15,7 @@
 		"@sveltejs/adapter-node": "^5.5.4",
 		"@sveltejs/kit": "^2.15.0",
 		"@sveltejs/vite-plugin-svelte": "^5.0.0",
+		"@types/bun": "^1.3.13",
 		"svelte": "^5.0.0",
 		"svelte-check": "^4.0.0",
 		"typescript": "^5.0.0",