feat(admin): multi-brand admin support

Implements the design at
docs/superpowers/specs/2026-06-04-multi-brand-admin-design.md.

Adds:
- admin_user_brands junction table (m:n admin<->brand) via migration 207
- New role 'multi_brand_admin' (auto-set when an admin has 2+ brands)
- 'active_brand_id' cookie that persists the admin's currently-selected
  brand across navigations; switchable via the new BrandSelector dropdown
  in the sidebar
- Centralised brand resolution in src/lib/brand-scope.ts:
  - getActiveBrandId (URL > cookie > legacy brand_id > first of brand_ids)
  - assertBrandAccess (defence-in-depth for cases where the brandId
    comes from a URL form or RPC return)
- ~30 server actions and ~10 page server components migrated to use
  getActiveBrandId instead of the silent brandId ?? adminUser.brand_id
  pattern that allowed cross-brand access bugs
- BrandSelector client component with proper a11y (aria-haspopup,
  aria-expanded, role=listbox, outside-click and escape-to-close)

Migration 207 also adds RLS so admins can read their own junction rows
(needed for the dropdown to populate) and SECURITY DEFINER RPCs
add/remove_admin_user_brand that auto-promote/demote between
brand_admin and multi_brand_admin.

Notes:
- Migration number is 207 not 204 — 204-206 were taken in this worktree
  by the concurrent locations work.
- The legacy admin_users.brand_id column is preserved for backwards
  compat; a follow-up migration 220_* will drop it.
- Dev sessions (dev_session cookie, NEXT_PUBLIC_USE_MOCK_DATA) get
  brand_ids: []; the documented limitation is that dev store_employee
  will see <AdminAccessDenied /> if no real brands exist.
- Pages that hardcode a Tuxedo brand UUID as a fallback
  (adminUser.brand_id ?? '64294306-...') are NOT migrated in this PR —
  they still work for single-brand admins and are out of scope.

Co-authored-by: implementer subagent (cancelled mid-run), Grok orchestrator

src/actions/water-log/admin.ts

@@ -1,6 +1,7 @@
 "use server";
 
 import { getAdminUser } from "@/lib/admin-permissions";
+import { getActiveBrandId } from "@/lib/brand-scope";
 import { svcHeaders } from "@/lib/svc-headers";
 
 type Irrigator = {
@@ -94,7 +95,7 @@ export async function updateWaterHeadgate(
         p_name: name,
         p_active: active,
         p_unit: unit ?? null,
-        p_brand_id: adminUser.brand_id ?? null,
+        p_brand_id: (await getActiveBrandId(adminUser)) ?? null,
         p_high_threshold: highThreshold ?? null,
         p_low_threshold: lowThreshold ?? null,
       }),
@@ -186,7 +187,7 @@ export async function updateWaterIrrigator(
         p_active: active,
         p_lang: lang,
         p_role: role,
-        p_brand_id: adminUser.brand_id ?? null,
+        p_brand_id: (await getActiveBrandId(adminUser)) ?? null,
       }),
     }
   );
@@ -215,7 +216,7 @@ export async function resetWaterIrrigatorPin(
       headers: { ...svcHeaders(supabaseKey), "Content-Type": "application/json" },
       body: JSON.stringify({
         p_user_id: irrigatorId,
-        p_brand_id: adminUser.brand_id ?? null,
+        p_brand_id: (await getActiveBrandId(adminUser)) ?? null,
       }),
     }
   );
@@ -242,7 +243,7 @@ export async function deleteWaterUser(userId: string): Promise<{ success: boolea
       headers: { ...svcHeaders(supabaseKey), "Content-Type": "application/json" },
       body: JSON.stringify({
         p_user_id: userId,
-        p_brand_id: adminUser.brand_id ?? null,
+        p_brand_id: (await getActiveBrandId(adminUser)) ?? null,
       }),
     }
   );
@@ -269,7 +270,7 @@ export async function deleteWaterHeadgate(headgateId: string): Promise<{ success
       headers: { ...svcHeaders(supabaseKey), "Content-Type": "application/json" },
       body: JSON.stringify({
         p_headgate_id: headgateId,
-        p_brand_id: adminUser.brand_id ?? null,
+        p_brand_id: (await getActiveBrandId(adminUser)) ?? null,
       }),
     }
   );
@@ -356,7 +357,7 @@ export async function regenerateHeadgateToken(headgateId: string): Promise<{ suc
     {
       method: "POST",
       headers: { ...svcHeaders(supabaseKey), "Content-Type": "application/json" },
-      body: JSON.stringify({ p_headgate_id: headgateId, p_brand_id: adminUser.brand_id ?? null }),
+      body: JSON.stringify({ p_headgate_id: headgateId, p_brand_id: (await getActiveBrandId(adminUser)) ?? null }),
     }
   );
 
@@ -392,7 +393,7 @@ export async function updateWaterEntry(
         p_measurement: measurement,
         p_notes: notes,
         p_unit: unit ?? null,
-        p_brand_id: adminUser.brand_id ?? null,
+        p_brand_id: (await getActiveBrandId(adminUser)) ?? null,
       }),
     }
   );
@@ -421,7 +422,7 @@ export async function deleteWaterEntry(
       headers: { ...svcHeaders(supabaseKey), "Content-Type": "application/json" },
       body: JSON.stringify({
         p_entry_id: entryId,
-        p_brand_id: adminUser.brand_id ?? null,
+        p_brand_id: (await getActiveBrandId(adminUser)) ?? null,
       }),
     }
   );