Domain isolation: typed contracts, service registry, and composable wiring

Add typed service contracts (Protocols + frozen DTOs) in shared/contracts/
for cross-domain communication. Each domain exposes a service interface
(BlogService, CalendarService, MarketService, CartService) backed by SQL
implementations in shared/services/. A singleton registry with has() guards
enables composable startup — apps register their own domain service and
stubs for absent domains.

Absorbs glue layer: navigation, relationships, event handlers (login,
container, order) now live in shared/ with has()-guarded service calls.
Factory gains domain_services_fn parameter for per-app service registration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

contracts/dtos.py

@@ -0,0 +1,113 @@
+"""Frozen dataclasses for cross-domain data transfer.
+
+These are the *only* shapes that cross domain boundaries.  Consumers never
+see ORM model instances from another domain — only these DTOs.
+"""
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from datetime import datetime
+from decimal import Decimal
+
+
+# ---------------------------------------------------------------------------
+# Blog domain
+# ---------------------------------------------------------------------------
+
+@dataclass(frozen=True, slots=True)
+class PostDTO:
+    id: int
+    slug: str
+    title: str
+    status: str
+    visibility: str
+    is_page: bool = False
+    feature_image: str | None = None
+    html: str | None = None
+    excerpt: str | None = None
+    custom_excerpt: str | None = None
+    published_at: datetime | None = None
+
+
+# ---------------------------------------------------------------------------
+# Calendar / Events domain
+# ---------------------------------------------------------------------------
+
+@dataclass(frozen=True, slots=True)
+class CalendarDTO:
+    id: int
+    container_type: str
+    container_id: int
+    name: str
+    slug: str
+    description: str | None = None
+
+
+@dataclass(frozen=True, slots=True)
+class CalendarEntryDTO:
+    id: int
+    calendar_id: int
+    name: str
+    start_at: datetime
+    state: str
+    cost: Decimal
+    end_at: datetime | None = None
+    user_id: int | None = None
+    session_id: str | None = None
+    order_id: int | None = None
+    slot_id: int | None = None
+    ticket_price: Decimal | None = None
+    ticket_count: int | None = None
+    calendar_name: str | None = None
+    calendar_slug: str | None = None
+
+
+# ---------------------------------------------------------------------------
+# Market domain
+# ---------------------------------------------------------------------------
+
+@dataclass(frozen=True, slots=True)
+class MarketPlaceDTO:
+    id: int
+    container_type: str
+    container_id: int
+    name: str
+    slug: str
+    description: str | None = None
+
+
+@dataclass(frozen=True, slots=True)
+class ProductDTO:
+    id: int
+    slug: str
+    title: str | None = None
+    image: str | None = None
+    description_short: str | None = None
+    rrp: Decimal | None = None
+    regular_price: Decimal | None = None
+    special_price: Decimal | None = None
+
+
+# ---------------------------------------------------------------------------
+# Cart domain
+# ---------------------------------------------------------------------------
+
+@dataclass(frozen=True, slots=True)
+class CartItemDTO:
+    id: int
+    product_id: int
+    quantity: int
+    product_title: str | None = None
+    product_slug: str | None = None
+    product_image: str | None = None
+    unit_price: Decimal | None = None
+    market_place_id: int | None = None
+
+
+@dataclass(frozen=True, slots=True)
+class CartSummaryDTO:
+    count: int = 0
+    total: Decimal = Decimal("0")
+    calendar_count: int = 0
+    calendar_total: Decimal = Decimal("0")
+    items: list[CartItemDTO] = field(default_factory=list)