Add claude init file + refactor some security issues

DEFERRED_SECURITY.md

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+# Deferred Security Issues
+
+These items were identified during a security review on 2026-05-02 and consciously deferred.
+**Must be addressed before opening the application to a larger or public userbase.**
+
+---
+
+## 1. Session credentials in URL query parameters (logged-in users)
+
+**Files:** `src/models/calendar/events/events.router.ts` — all GET/PUT/DELETE handlers
+
+`sessionId` and `sessionKey` are currently read from query parameters, which means they appear in server access logs, browser history, proxy logs, and `Referer` headers.
+
+**Fix:** Move to request headers (`X-Session-Id` / `X-Session-Key`) or the request body. Requires a corresponding frontend update.
+
+> Note: the shared calendar `password` parameter in query params is intentional (iCal clients don't support headers) and is acceptable for the current setup.
+
+---
+
+## 2. No event ownership check
+
+**Files:** `src/models/calendar/events/events.router.ts`
+- `PUT /:eventId` (update)
+- `PUT /move/:eventId` (move)
+- `DELETE /:eventId` (delete)
+
+Currently any active user can edit, move, or delete any event regardless of who created it. This is acceptable while all users are trusted admins.
+
+**Fix:** When non-admin users are introduced, fetch the event first and verify `event.createdById === user.userId` before allowing the mutation. Add an `isAdmin` flag to the user model to let admins bypass the check.
+
+---
+
+## 3. Activation token has no expiry
+
+**File:** `src/models/calendar/users/users.service.ts` — `createUser` / `activateUser`
+
+The email activation link is valid indefinitely. Acceptable for a small, trusted userbase.
+
+**Fix:**
+1. Add an `activation_expires` column to the `users` table (e.g. `DATETIME`).
+2. Set it to `NOW() + INTERVAL 24 HOUR` in `createUser`.
+3. Check `activation_expires > NOW()` in `activateUser` before accepting the token.
+
+---
+
+## 4. Password reset token has no expiry
+
+**File:** `src/models/calendar/users/users.service.ts` — `initiatePasswordReset` / `finalizePasswordReset`
+
+The reset token stored in `pw_reset_token_hash` never expires. Acceptable for a small, trusted userbase.
+
+**Fix:**
+1. Add a `pw_reset_expires` column to the `users` table (e.g. `DATETIME`).
+2. Set it to `NOW() + INTERVAL 15 MINUTE` in `initiatePasswordReset`.
+3. Check `pw_reset_expires > NOW()` in `finalizePasswordReset` before accepting the token.