docs: pokyn pro zápis nových funkcí do changelogu

Přidána sekce do CLAUDE.md, která vyžaduje zaznamenání uživatelsky viditelných změn do server/changelogs/YYYY-MM-DD.json.

CLAUDE.md

@@ -126,3 +126,21 @@ Prettier is installed in `client/` (devDependency only, no script or config) —
 - TypeScript strict mode in both client and server
 - Server module resolution: Node16; Client: ESNext/bundler
 - `TODO.md` tracks open bugs and roadmap items — worth scanning before starting non-trivial work
+
+## Changelog (user-facing "What's new")
+
+When you add or change a user-visible feature, you MUST also record it in `server/changelogs/` so users see it via the in-app changelog (served by `server/src/routes/changelogRoutes.ts`, sorted newest-first with a `?since=` filter).
+
+Steps:
+
+1. Create or open the file for today's date: `server/changelogs/YYYY-MM-DD.json` (use today's date; one file per day).
+2. The file content is a **JSON array of Czech strings**, one short user-facing sentence per change. Example:
+   ```json
+   [
+     "Proklik na nabídku podniku ze stránky objednávek",
+     "Možnost přidat URL pro sledování stavu doručení pro Bolt Food"
+   ]
+   ```
+3. If a file for today already exists, append your entry to its array instead of creating a new file.
+4. Write entries in Czech, phrased for end users (describe the benefit, not the implementation). Skip purely internal/refactor changes that users won't notice.
+5. The route caches files in memory at read time, so a server restart is needed to pick up new/changed changelogs in a running dev instance.