security: fix 10 vulnerabilities from security audit

- SSRF: block private/internal URLs in image cache fetch
- SSRF: block private/internal URLs in RSS feed fetch (scheme guard)
- SSRF: block private/internal URLs in CalDAV URL setting
- Auth: require login for GET /api/images/<id> (was unauthenticated)
- Auth: restrict Ollama model pull/delete to admin users only
- Info disclosure: remove email from /api/users/search response
- OAuth: skip email-based account linking when email_verified is false
- Config: raise hard error on default SECRET_KEY when SECURE_COOKIES=true
- Rate limit: document proxy header requirement; add startup warning
- XSS: remove src/alt from global DOMPurify ADD_ATTR allowlist

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/ShareDialog.vue

@@ -138,7 +138,6 @@ onMounted(async () => {
               <ul v-if="userResults.length" class="user-results">
                 <li v-for="u in userResults" :key="u.id" @click="selectUser(u)" class="user-result-item">
                   <span class="user-result-name">{{ u.username }}</span>
-                  <span class="user-result-email">{{ u.email }}</span>
                 </li>
               </ul>
             </div>