Add locations (Filialen) as a grouping layer above resources

Enables multiple barbers/staff bookable at the same location and time
-- previously "resource" conflated "location" and "the thing that
can't double-book itself" into one row, so a Filiale could only ever
have exactly one bookable slot at once.

- New `locations` table; `resources.location_id` with a generic,
  idempotent backfill migration (any resource without a location gets
  one auto-created matching its name -- not a one-off for any single
  client, protects any future resource stuck in the old flat shape too)
- `resources`/`resource_hours`/services keep everything they already
  had (hours, min-notice, max-advance, buffer, the no-overlap
  constraint) scoped to resource_id, not location_id -- two barbers at
  one location must stay independently bookable at the same time
- booking_db.py: new locations CRUD mirroring the existing
  resources/services pattern; create_resource now requires a
  location_id, guarded the same way every other tenant check here is
  (get_location existence check, no real FK -- matches this schema's
  existing no-FK convention throughout)
- app.py: new POST /api/locations provisioning route; POST
  /api/resources now requires location_id
- owner_settings.py + settings.html: new self-service "add a Filiale"
  / "add a barber" UI -- there was previously no way to create a
  resource at all outside the CRM/n8n provisioning API
- public_booking.py + book.html: new Filiale picker (reuses the
  existing wireOptionGroup button-group pattern), filtering the
  Mitarbeiter picker to the selected location -- a single-location
  client sees no extra click, same as before Filialen existed
- owner_booking.py + agenda.html: the Filiale show/hide toggle and
  hide-cancelled toggle (shipped earlier this session) now key off
  location_id instead of resource_id, so hiding a Filiale hides every
  barber's bookings at it; manual-booking dropdown grouped by Filiale
- n8n/onboarding.json: default provisioning now creates a "Hauptfiliale"
  location before its resource (inert until re-imported into the live
  n8n instance)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

n8n/onboarding.json

@@ -133,7 +133,7 @@
     },
     {
       "parameters": {
-        "jsCode": "const cid=$('Save client').first().json.added;\nconst c=($('Compute').first().json.client)||{};\nconst password=Math.random().toString(36).slice(2,10)+Math.random().toString(36).slice(2,6);\nconst hours=[0,1,2,3,4,5].map(weekday=>({weekday,opens_at:'09:00',closes_at:'18:00'}));\nconst resourceBody={client_id:cid,name:'Hauptressource',hours};\nconst serviceBody={client_id:cid,name:'Termin',duration_minutes:30,price:0};\nreturn [{json:{cid,resourceBody,serviceBody,userEmail:c.email,userPassword:password}}];"
+        "jsCode": "const cid=$('Save client').first().json.added;\nconst c=($('Compute').first().json.client)||{};\nconst password=Math.random().toString(36).slice(2,10)+Math.random().toString(36).slice(2,6);\nconst hours=[0,1,2,3,4,5].map(weekday=>({weekday,opens_at:'09:00',closes_at:'18:00'}));\nconst locationBody={client_id:cid,name:'Hauptfiliale'};\nconst resourceBody={client_id:cid,name:'Hauptressource',hours};\nconst serviceBody={client_id:cid,name:'Termin',duration_minutes:30,price:0};\nreturn [{json:{cid,locationBody,resourceBody,serviceBody,userEmail:c.email,userPassword:password}}];"
       },
       "id": "prov-build-provisioning",
       "name": "Build provisioning",
@@ -144,6 +144,37 @@
         520
       ]
     },
+    {
+      "parameters": {
+        "options": {},
+        "method": "POST",
+        "url": "http://smb-crm:8080/api/locations",
+        "sendHeaders": true,
+        "headerParameters": {
+          "parameters": [
+            {
+              "name": "X-CRM-Token",
+              "value": "__CRM_TOKEN__"
+            }
+          ]
+        },
+        "sendBody": true,
+        "specifyBody": "json",
+        "jsonBody": "={{ JSON.stringify($json.locationBody) }}"
+      },
+      "id": "prov-create-location",
+      "name": "Create location",
+      "type": "n8n-nodes-base.httpRequest",
+      "typeVersion": 4.2,
+      "position": [
+        900,
+        340
+      ],
+      "retryOnFail": true,
+      "maxTries": 3,
+      "waitBetweenTries": 2000,
+      "continueOnFail": true
+    },
     {
       "parameters": {
         "options": {},
@@ -160,14 +191,14 @@
         },
         "sendBody": true,
         "specifyBody": "json",
-        "jsonBody": "={{ JSON.stringify($json.resourceBody) }}"
+        "jsonBody": "={{ JSON.stringify(Object.assign({}, $('Build provisioning').item.json.resourceBody, { location_id: $json.location_id })) }}"
       },
       "id": "prov-create-resource",
       "name": "Create resource",
       "type": "n8n-nodes-base.httpRequest",
       "typeVersion": 4.2,
       "position": [
-        900,
+        1120,
         420
       ],
       "retryOnFail": true,
@@ -367,7 +398,7 @@
       "main": [
         [
           {
-            "node": "Create resource",
+            "node": "Create location",
             "type": "main",
             "index": 0
           },
@@ -384,6 +415,17 @@
         ]
       ]
     },
+    "Create location": {
+      "main": [
+        [
+          {
+            "node": "Create resource",
+            "type": "main",
+            "index": 0
+          }
+        ]
+      ]
+    },
     "Create owner user": {
       "main": [
         [