feat: Add password login system and upgrade agent models

- App-level password auth via Flask session (APP_PASSWORD in .env)
- login_required decorator on all routes
- Login page, logout button in navbar, 7-day session lifetime
- Upgrade musik_rechte_advisor and negotiator from Opus 4.0 to Opus 4.6
- Fix orchestrator session cookie overflow (kb/prompts no longer stored in session)
- Change app port from 5000 to 5050 (5000 occupied by Zou/Kitsu)

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

agent_config.json

@@ -15,7 +15,7 @@
     "model": "anthropic/claude-sonnet-4-6"
   },
   "musik_rechte_advisor": {
-    "model": "anthropic/claude-opus-4-0"
+    "model": "anthropic/claude-opus-4-6"
   },
   "zusammenfasser": {
     "model": "anthropic/claude-haiku-4-5"
@@ -24,7 +24,7 @@
     "model": "anthropic/claude-haiku-4-5"
   },
   "negotiator": {
-    "model": "anthropic/claude-opus-4-0"
+    "model": "anthropic/claude-opus-4-6"
   },
   "budget_manager": {
     "model": "anthropic/claude-haiku-4-5"