feat: enhance booking flow with BPF wizard, weekend pricing, and document uploads

- Updated translations for German and English to reflect changes in deposit terminology.
- Modified index.html to implement a new booking flow with a step-by-step wizard for vehicle selection, contact details, and ID verification.
- Added CSS styles for the new booking flow interface, including responsive design and improved input styles.
- Created new database policies and tables for handling customer and lead attachments during the booking process.
- Introduced weekend pricing and daily KM limits for vehicles in the database schema.
- Implemented email-based customer upsert functionality to streamline lead qualification and attachment transfer.

Co-authored-by: Copilot <copilot@github.com>

frontend/admin.html

@@ -158,13 +158,14 @@
                         <div class="row3">
                             <label><span data-i18n="adminSeats">Sitze</span><input type="number" name="seats" min="1" value="2" /></label>
                             <label><span data-i18n="adminPrice">Preis / Tag (€)</span><input type="number" name="daily_price_eur" min="0" required /></label>
-                            <label><span data-i18n="adminSort">Reihenfolge</span><input type="number" name="sort_order" value="100" /></label>
+                            <label><span>Wochenendpreis (€)</span><input type="number" name="weekend_price_eur" min="0" /></label>
                         </div>
 
-                        <label>
-                            <span data-i18n="adminLocation">Standort</span>
-                            <input name="location" value="Steiermark (TBD)" />
-                        </label>
+                        <div class="row3">
+                            <label><span>Max. km/Tag</span><input type="number" name="max_daily_km" min="0" value="150" /></label>
+                            <label><span data-i18n="adminSort">Reihenfolge</span><input type="number" name="sort_order" value="100" /></label>
+                            <label><span data-i18n="adminLocation">Standort</span><input name="location" value="Steiermark (TBD)" /></label>
+                        </div>
 
                         <label>
                             <span data-i18n="adminDescDe">Beschreibung (Deutsch)</span>