feat(A5): add app_config table + typed config service

- Migration 034: creates app_config table with JSONB columns (render, storage,
  worker, notifications, billing); migrates existing system_settings values
- backend/app/core/config_service.py:
  - Typed Pydantic models: RenderConfig, WorkerConfig, StorageConfig, etc.
  - AppConfig aggregate model
  - get_app_config(db) async + get_app_config_sync() for Celery tasks
  - update_render_config() / update_worker_config() for partial updates
- system_settings table preserved for backward compat during Phase B migration

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

backend/app/core/config_service.py

@@ -0,0 +1,168 @@
+"""AppConfig service — typed access to structured application settings.
+
+Replaces the key-value system_settings table with JSONB-column app_config.
+Both old and new APIs are supported during migration; system_settings is kept
+as a read-only backward-compat layer.
+
+Usage:
+    from app.core.config_service import get_app_config, update_render_config
+
+    # Async (FastAPI handlers)
+    config = await get_app_config(db)
+    config.render.thumbnail_renderer  # → "blender"
+
+    # Sync (Celery tasks)
+    from app.core.config_service import get_app_config_sync
+    config = get_app_config_sync()
+"""
+import json
+import logging
+from datetime import datetime
+from typing import Any
+
+from pydantic import BaseModel, Field
+from sqlalchemy import text
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import Session
+
+logger = logging.getLogger(__name__)
+
+
+# ---------------------------------------------------------------------------
+# Typed config models
+# ---------------------------------------------------------------------------
+
+class RenderConfig(BaseModel):
+    thumbnail_renderer: str = "blender"
+    blender_engine: str = "cycles"
+    blender_cycles_samples: int = 256
+    blender_eevee_samples: int = 64
+    thumbnail_format: str = "jpg"
+    stl_quality: str = "low"
+    blender_smooth_angle: int = 30
+    cycles_device: str = "auto"
+    render_backend: str = "celery"
+    product_thumbnail_priority: list[str] = Field(
+        default_factory=lambda: ["latest_render", "cad_thumbnail"]
+    )
+
+
+class WorkerConfig(BaseModel):
+    concurrency: int = 8
+    max_concurrent_renders: int = 3
+    render_stall_timeout_minutes: int = 120
+
+
+class StorageConfig(BaseModel):
+    upload_dir: str = "/app/uploads"
+    max_upload_size_mb: int = 500
+
+
+class NotificationsConfig(BaseModel):
+    pass  # reserved for Phase G
+
+
+class BillingConfig(BaseModel):
+    pass  # reserved for Phase L
+
+
+class AppConfig(BaseModel):
+    render: RenderConfig = Field(default_factory=RenderConfig)
+    storage: StorageConfig = Field(default_factory=StorageConfig)
+    notifications: NotificationsConfig = Field(default_factory=NotificationsConfig)
+    worker: WorkerConfig = Field(default_factory=WorkerConfig)
+    billing: BillingConfig = Field(default_factory=BillingConfig)
+    updated_at: datetime | None = None
+
+
+# ---------------------------------------------------------------------------
+# DB access
+# ---------------------------------------------------------------------------
+
+_SELECT_SQL = "SELECT render, storage, notifications, worker, billing, updated_at FROM app_config ORDER BY version DESC LIMIT 1"
+_DEFAULT_CONFIG = AppConfig()
+
+
+async def get_app_config(db: AsyncSession) -> AppConfig:
+    """Load AppConfig from app_config table (async)."""
+    try:
+        result = await db.execute(text(_SELECT_SQL))
+        row = result.one_or_none()
+        return _row_to_config(row)
+    except Exception as exc:
+        logger.warning("get_app_config failed, returning defaults: %s", exc)
+        return _DEFAULT_CONFIG
+
+
+def get_app_config_sync(session: Session | None = None) -> AppConfig:
+    """Load AppConfig from app_config table (sync, for Celery tasks)."""
+    try:
+        if session:
+            row = session.execute(text(_SELECT_SQL)).one_or_none()
+            return _row_to_config(row)
+
+        from app.config import settings as app_settings
+        from sqlalchemy import create_engine
+        engine = create_engine(app_settings.database_url_sync)
+        with Session(engine) as s:
+            row = s.execute(text(_SELECT_SQL)).one_or_none()
+        engine.dispose()
+        return _row_to_config(row)
+    except Exception as exc:
+        logger.warning("get_app_config_sync failed, returning defaults: %s", exc)
+        return _DEFAULT_CONFIG
+
+
+async def update_render_config(db: AsyncSession, updates: dict[str, Any]) -> AppConfig:
+    """Merge updates into the render section of app_config."""
+    return await _update_section(db, "render", updates)
+
+
+async def update_worker_config(db: AsyncSession, updates: dict[str, Any]) -> AppConfig:
+    """Merge updates into the worker section of app_config."""
+    return await _update_section(db, "worker", updates)
+
+
+async def _update_section(db: AsyncSession, section: str, updates: dict[str, Any]) -> AppConfig:
+    valid_sections = {"render", "storage", "notifications", "worker", "billing"}
+    if section not in valid_sections:
+        raise ValueError(f"Invalid config section: {section}")
+
+    await db.execute(
+        text(f"""
+            UPDATE app_config
+            SET {section} = {section} || :patch::jsonb,
+                updated_at = NOW()
+        """),
+        {"patch": json.dumps(updates)},
+    )
+    await db.commit()
+    return await get_app_config(db)
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+def _row_to_config(row) -> AppConfig:
+    if row is None:
+        return _DEFAULT_CONFIG
+
+    def _parse(raw, model_cls):
+        if raw is None:
+            return model_cls()
+        if isinstance(raw, str):
+            raw = json.loads(raw)
+        try:
+            return model_cls(**raw)
+        except Exception:
+            return model_cls()
+
+    return AppConfig(
+        render=_parse(row[0], RenderConfig),
+        storage=_parse(row[1], StorageConfig),
+        notifications=_parse(row[2], NotificationsConfig),
+        worker=_parse(row[3], WorkerConfig),
+        billing=_parse(row[4], BillingConfig),
+        updated_at=row[5],
+    )