feat: Phase 2 – Budget-Engine + Direktzugriff (http://IP:Port)

Budget:
- Modelle + Migration 0003: budget_template_lines, planned_income_lines,
  budget_periods, budget_period_lines.
- services/budget: Perioden-Grenzen (einstellbarer Starttag, Label nach Endmonat),
  Standard-Vorlage vs. Monats-Override (Snapshot-on-Customize), Soll/Ist inkl.
  budgetierter Sparen-Transfers, geplante Einnahmen (Soll/Ist).
- routers/budgets: Vorlage, geplante Einnahmen (CRUD), Monats-Soll/Ist,
  Monats-Override, Zurücksetzen. pytest: Perioden, Soll/Ist, Override/Reset, Einnahmen.
- Frontend: Budget-Seite (Monat + Standard-Budget), Fortschrittsbalken grün/rot,
  Monats-Navigation, editierbare Soll-Beträge; Sparen-Option im Transfer-Formular.

Deployment-Fix (Nutzer-Anforderung: http://IP:Port muss funktionieren):
- Dienst bindet an APP_BIND_HOST=0.0.0.0 (direkt im LAN erreichbar).
- APP_COOKIE_SECURE=false als Standard (sonst kein Login über reines HTTP);
  für reinen HTTPS-Betrieb umstellbar. Doku + install.sh entsprechend angepasst.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/App.jsx

@@ -10,6 +10,7 @@ import Dashboard from "./pages/Dashboard";
 import Accounts from "./pages/Accounts";
 import Categories from "./pages/Categories";
 import Transactions from "./pages/Transactions";
+import Budget from "./pages/Budget";
 import Users from "./pages/Users";
 import Settings from "./pages/Settings";
 
@@ -44,6 +45,7 @@ function Sidebar() {
         <NavItem to="/konten" icon="wallet" label={t("nav.accounts")} />
         <NavItem to="/kategorien" icon="list" label={t("nav.categories")} />
         <NavItem to="/buchungen" icon="arrow-left-right" label={t("nav.transactions")} />
+        <NavItem to="/budget" icon="pie-chart" label={t("nav.budget")} />
         <NavItem to="/settings" icon="settings" label={t("nav.settings")} />
         {isAdmin && (
           <>
@@ -128,6 +130,7 @@ export default function App() {
       <Route path="/konten" element={<Protected><Accounts /></Protected>} />
       <Route path="/kategorien" element={<Protected><Categories /></Protected>} />
       <Route path="/buchungen" element={<Protected wide><Transactions /></Protected>} />
+      <Route path="/budget" element={<Protected wide><Budget /></Protected>} />
       <Route path="/settings" element={<Protected><Settings /></Protected>} />
       <Route path="/users" element={<Protected adminOnly><Users /></Protected>} />
       <Route path="*" element={<Navigate to="/" replace />} />