Replace default_model with force_model (model lock)

Removes DEFAULT_MODEL in favour of a force_model setting configurable
via the admin UI. When set, every proxy request's model field is
overridden, preventing uncoordinated model switches during lab sessions.
Updates schemas, admin API, all three proxy endpoints, frontend,
init_db, and docs (README, DOCKERHUB, KURZANLEITUNG).

frontend/src/main.jsx

@@ -95,8 +95,8 @@ function SettingsSection({ password }) {
       const { models, reachable } = res.data;
       setOllamaReachable(reachable);
       setAvailableModels(models);
-      if (models.length > 0 && !models.includes(currentModel)) {
-        setSettings(s => ({ ...s, default_model: models[0] }));
+      if (models.length > 0 && currentModel && !models.includes(currentModel)) {
+        setSettings(s => ({ ...s, force_model: models[0] }));
       }
     } catch {
       setOllamaReachable(false);
@@ -115,7 +115,7 @@ function SettingsSection({ password }) {
       const s = settingsRes.data;
       setSettings(s);
       setProxyEndpoint(proxyRes.data.endpoint);
-      fetchModels(s.ollama_url, s.default_model);
+      fetchModels(s.ollama_url, s.force_model);
     }).catch(() => setError('Einstellungen konnten nicht geladen werden.'));
   }, []);
 
@@ -152,7 +152,7 @@ function SettingsSection({ password }) {
               type="url"
               value={settings.ollama_url}
               onChange={(e) => setSettings({ ...settings, ollama_url: e.target.value })}
-              onBlur={(e) => fetchModels(e.target.value, settings.default_model)}
+              onBlur={(e) => fetchModels(e.target.value, settings.force_model)}
               placeholder="http://localhost:11434"
               required
             />
@@ -162,23 +162,23 @@ function SettingsSection({ password }) {
           </div>
         </div>
         <div className="settings-row">
-          <label>Standard-Modell</label>
+          <label>Aktives Modell (Lock)</label>
           {modelsLoading ? (
             <span className="settings-value">Lade Modelle…</span>
           ) : availableModels.length > 0 ? (
             <select
-              value={settings.default_model}
-              onChange={(e) => setSettings({ ...settings, default_model: e.target.value })}
+              value={settings.force_model || ""}
+              onChange={(e) => setSettings({ ...settings, force_model: e.target.value || null })}
             >
+              <option value="">— kein Lock —</option>
               {availableModels.map(m => <option key={m} value={m}>{m}</option>)}
             </select>
           ) : (
             <input
               type="text"
-              value={settings.default_model}
-              onChange={(e) => setSettings({ ...settings, default_model: e.target.value })}
-              placeholder="llama3"
-              required
+              value={settings.force_model || ""}
+              onChange={(e) => setSettings({ ...settings, force_model: e.target.value || null })}
+              placeholder="leer = kein Lock"
             />
           )}
         </div>