PC3 Phase 1.3: backend avatar loadout persistence

New avatar_loadouts table (one row per user, loadout stored as a single
JSONB document matching the client's AvatarLoadout.to_json() shape) and
GET/PUT /api/avatar/loadout.

The server has no copy of the asset catalog, so it validates only the
envelope (JSON object, non-empty base_body, object slots, size cap) --
which-asset-in-which-slot validation stays client-side. New accounts get a
server default (base_human_male_01, no slots) with is_default:true so the
client can tell "never customised" from "customised back to base".

- models/avatar_loadout.py + 0004 migration + User.avatar_loadout relation
- routers/avatar.py wired into main.py
- tests/test_avatar_loadout.py (7 tests: default, save/read, replace,
  validation, auth) -- full suite 68 passed
- lifted the duplicated _override_db fixture from 3 test files into
  conftest.py (new test files need it too)
- docs/architecture/avatar.md §9a: the persistence design

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

backend/app/main.py

@@ -6,7 +6,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.config import settings
-from app.routers import auth, users, worlds, assets
+from app.routers import auth, users, worlds, assets, avatar
 
 
 @asynccontextmanager
@@ -38,6 +38,7 @@ app.include_router(auth.router, prefix="/api/auth", tags=["auth"])
 app.include_router(users.router, prefix="/api/users", tags=["users"])
 app.include_router(worlds.router, prefix="/api/worlds", tags=["worlds"])
 app.include_router(assets.router, prefix="/api/assets", tags=["assets"])
+app.include_router(avatar.router, prefix="/api/avatar", tags=["avatar"])
 
 
 @app.get("/api/health")

backend/app/models/__init__.py

@@ -2,5 +2,6 @@ from app.models.user import User
 from app.models.session import Session
 from app.models.world import World
 from app.models.asset import Asset
+from app.models.avatar_loadout import AvatarLoadout
 
-__all__ = ["User", "Session", "World", "Asset"]
+__all__ = ["User", "Session", "World", "Asset", "AvatarLoadout"]

backend/app/models/avatar_loadout.py

@@ -0,0 +1,41 @@
+from sqlalchemy import Column, Integer, ForeignKey, DateTime
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+
+from app.database import Base
+
+
+# The default a brand-new account gets before they touch the customization
+# screen. Kept in sync with assets/avatars/catalog.json (the base_body id) and
+# with the client's AvatarLoadout.to_json() shape.
+DEFAULT_LOADOUT: dict = {
+    "base_body": "base_human_male_01",
+    "slots": {},
+}
+
+
+class AvatarLoadout(Base):
+    """A user's equipped avatar composition.
+
+    One row per user. The whole loadout is stored as a single JSON document
+    matching the client's ``AvatarLoadout.to_json()`` shape
+    (``{"base_body": str, "slots": {...}, "skin_tone"?: {...}}``) rather than
+    exploded into columns -- the server has no copy of the asset catalog, so
+    deep validation (does this asset exist, is it valid for this slot) stays
+    on the client. The server only enforces the envelope: valid JSON object
+    with a non-empty ``base_body``.
+    """
+
+    __tablename__ = "avatar_loadouts"
+
+    user_id = Column(
+        Integer,
+        ForeignKey("users.id", ondelete="CASCADE"),
+        primary_key=True,
+    )
+    data = Column(JSONB, nullable=False)
+    created_at = Column(DateTime(timezone=True), server_default=func.now(), nullable=False)
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now(), nullable=False)
+
+    user = relationship("User", back_populates="avatar_loadout")

backend/app/models/user.py

@@ -24,6 +24,12 @@ class User(Base):
     refresh_tokens = relationship("RefreshToken", back_populates="user", cascade="all, delete-orphan")
     worlds = relationship("World", back_populates="owner")
     assets = relationship("Asset", back_populates="owner")
+    avatar_loadout = relationship(
+        "AvatarLoadout",
+        back_populates="user",
+        uselist=False,
+        cascade="all, delete-orphan",
+    )
 
     @property
     def is_adult(self) -> bool:

backend/app/routers/avatar.py

@@ -0,0 +1,78 @@
+"""Avatar router - the current user's persisted avatar loadout.
+
+The loadout is a single JSON document matching the client's
+``AvatarLoadout.to_json()`` shape. The server has no copy of the asset
+catalog, so it only validates the envelope (JSON object, non-empty
+``base_body``); which assets are valid for which slot is the client's job.
+"""
+
+from typing import Annotated, Any
+
+from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel, Field
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.database import get_db
+from app.dependencies import CurrentUser
+from app.models.avatar_loadout import AvatarLoadout, DEFAULT_LOADOUT
+
+router = APIRouter()
+
+DbDep = Annotated[AsyncSession, Depends(get_db)]
+
+# A loadout document is small; reject anything absurd outright.
+MAX_LOADOUT_BYTES = 16 * 1024
+
+
+class LoadoutResponse(BaseModel):
+    data: dict[str, Any]
+    is_default: bool = Field(
+        description="True when the user has never saved a loadout and this is "
+        "the server default."
+    )
+
+
+class LoadoutUpdate(BaseModel):
+    data: dict[str, Any]
+
+
+def _validate_envelope(data: dict[str, Any]) -> None:
+    base_body = data.get("base_body")
+    if not isinstance(base_body, str) or not base_body.strip():
+        raise HTTPException(
+            status_code=422,
+            detail="loadout.data must have a non-empty string 'base_body'",
+        )
+    slots = data.get("slots", {})
+    if not isinstance(slots, dict):
+        raise HTTPException(status_code=422, detail="loadout.data.slots must be an object")
+    if len(str(data)) > MAX_LOADOUT_BYTES:
+        raise HTTPException(status_code=413, detail="loadout document too large")
+
+
+@router.get("/loadout", response_model=LoadoutResponse)
+async def get_loadout(current_user: CurrentUser, db: DbDep):
+    """Return the current user's saved loadout, or the server default if they
+    have never saved one."""
+    row = await db.get(AvatarLoadout, current_user.id)
+    if row is None:
+        return LoadoutResponse(data=dict(DEFAULT_LOADOUT), is_default=True)
+    return LoadoutResponse(data=row.data, is_default=False)
+
+
+@router.put("/loadout", response_model=LoadoutResponse)
+async def put_loadout(payload: LoadoutUpdate, current_user: CurrentUser, db: DbDep):
+    """Replace the current user's loadout."""
+    _validate_envelope(payload.data)
+
+    row = await db.get(AvatarLoadout, current_user.id)
+    if row is None:
+        row = AvatarLoadout(user_id=current_user.id, data=payload.data)
+        db.add(row)
+    else:
+        row.data = payload.data
+
+    await db.commit()
+    await db.refresh(row)
+    return LoadoutResponse(data=row.data, is_default=False)