docs(architecture): ADR-011 - optional Seller Management module

Documents the decision behind the typed contracts added in the
previous commit: Seller Management is an optional platform capability
module (Platform -> Marketplace -> Seller, 0..N per marketplace), not
a second tenancy tier. Backend resolves seller scope the same way it
already resolves tenant (ADR-001); frontend never resolves it itself.
Gated by one typed flag (modules.sellerManagement.enabled), same
capability-guard discipline as ADR-009, defaulting to disabled/absent
so existing marketplaces are byte-identical.

Explicitly scopes out UI, backend, and business logic as future work
requiring its own ADR/implementation pass once the module is actually
built out.

Added companion diagrams (Seller-Management-Diagrams.md): hierarchy,
bootstrap module-gate flow, and the type-contract class diagram.
Registered ADR-011 in the foundation README's ADR index.

docs/architecture/foundation/Seller-Management-Diagrams.md

@@ -0,0 +1,72 @@
+# Seller Management — Architecture Diagrams
+
+Companion diagrams for [ADR-011](adr/ADR-011-optional-seller-management-module.md).
+Architecture only — no UI, no backend, no business logic exists yet.
+
+## 1. Hierarchy
+
+```mermaid
+graph TD
+    Platform["Platform<br/>(one Angular runtime)"]
+    Marketplace["Marketplace (tenant)<br/>always present · backend-resolved from Host<br/>ADR-001"]
+    SellerA["Seller A<br/>optional, 0..N"]
+    SellerB["Seller B<br/>optional, 0..N"]
+    NoSeller["No sellers<br/>(default — most marketplaces today)"]
+
+    Platform --> Marketplace
+    Marketplace --> SellerA
+    Marketplace --> SellerB
+    Marketplace -.default state.-> NoSeller
+```
+
+Marketplace is the only primary tenant. Seller is a child scope of exactly
+one marketplace — never a sibling tier, never resolved on its own.
+
+## 2. Bootstrap module gate
+
+```mermaid
+graph LR
+    Request["GET /bootstrap"] --> Backend["Backend resolves:<br/>tenant (always)<br/>seller (only if applicable)"]
+    Backend --> Bootstrap["BootstrapConfig"]
+    Bootstrap --> ModulesCheck{"modules.sellerManagement.enabled?"}
+    ModulesCheck -->|false / absent, default| Identical["Behavior identical to today.<br/>No new routes, menus, or API calls."]
+    ModulesCheck -->|true| Available["Seller-aware behavior becomes available<br/>(not built yet — future work, own ADR)"]
+    Bootstrap -.optional field.-> SellerField["BootstrapConfig.seller<br/>(SellerConfig, present only when<br/>backend resolved a seller scope)"]
+```
+
+The frontend performs no resolution — it reads whatever the backend already
+decided into `BootstrapConfig.modules` / `BootstrapConfig.seller`, exactly
+the same discipline as tenant resolution (ADR-001) and feature-flag gating
+(ADR-009).
+
+## 3. Type contracts introduced (this ADR only)
+
+```mermaid
+classDiagram
+    class BootstrapConfig {
+        +TenantConfig tenant
+        +PlatformModulesConfig? modules
+        +SellerConfig? seller
+        ...existing fields unchanged
+    }
+    class PlatformModulesConfig {
+        +SellerManagementModuleConfig sellerManagement
+    }
+    class SellerManagementModuleConfig {
+        +boolean enabled
+    }
+    class SellerConfig {
+        +UUID id
+        +UUID marketplaceId
+        +string slug
+        +string name
+        +string defaultLocale
+        +string[] supportedLocales
+    }
+    BootstrapConfig --> PlatformModulesConfig
+    BootstrapConfig --> SellerConfig
+    PlatformModulesConfig --> SellerManagementModuleConfig
+```
+
+`modules` and `seller` are both optional on `BootstrapConfig`. Every field
+already on `BootstrapConfig` is untouched.