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/alembic/versions/034_app_config.py

@@ -0,0 +1,92 @@
+"""Add app_config table (structured settings to replace key-value system_settings).
+
+app_config stores settings grouped in JSONB columns:
+  - render: thumbnail_renderer, engine, samples, stl_quality, etc.
+  - storage: upload_dir, minio_url, max_upload_size_mb, etc.
+  - notifications: (reserved for Phase G)
+  - worker: concurrency, max_concurrent_renders, stall_timeout_minutes
+  - billing: (reserved for Phase L)
+
+system_settings table is preserved for backward compatibility during migration.
+Existing values are migrated into app_config.
+
+Revision ID: 034
+Revises: 033
+Create Date: 2026-03-06
+"""
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects.postgresql import JSONB, UUID
+
+revision = '034'
+down_revision = '033'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.create_table(
+        'app_config',
+        sa.Column('id', UUID(as_uuid=True), primary_key=True,
+                  server_default=sa.text('gen_random_uuid()')),
+        sa.Column('version', sa.Integer(), nullable=False, server_default='1'),
+        sa.Column('render', JSONB(), nullable=False, server_default='{}'),
+        sa.Column('storage', JSONB(), nullable=False, server_default='{}'),
+        sa.Column('notifications', JSONB(), nullable=False, server_default='{}'),
+        sa.Column('worker', JSONB(), nullable=False, server_default='{}'),
+        sa.Column('billing', JSONB(), nullable=False, server_default='{}'),
+        sa.Column('updated_at', sa.DateTime(), nullable=False,
+                  server_default=sa.text('NOW()')),
+    )
+
+    # Migrate existing system_settings into app_config render + worker columns
+    op.execute("""
+        INSERT INTO app_config (render, worker, updated_at)
+        SELECT
+            jsonb_build_object(
+                'thumbnail_renderer', COALESCE(MAX(CASE WHEN key = 'thumbnail_renderer' THEN value END), 'blender'),
+                'blender_engine',     COALESCE(MAX(CASE WHEN key = 'blender_engine' THEN value END), 'cycles'),
+                'blender_cycles_samples', COALESCE(MAX(CASE WHEN key = 'blender_cycles_samples' THEN value END), '256')::int,
+                'blender_eevee_samples',  COALESCE(MAX(CASE WHEN key = 'blender_eevee_samples' THEN value END), '64')::int,
+                'thumbnail_format',   COALESCE(MAX(CASE WHEN key = 'thumbnail_format' THEN value END), 'jpg'),
+                'stl_quality',        COALESCE(MAX(CASE WHEN key = 'stl_quality' THEN value END), 'low'),
+                'blender_smooth_angle', COALESCE(MAX(CASE WHEN key = 'blender_smooth_angle' THEN value END), '30')::int,
+                'cycles_device',      COALESCE(MAX(CASE WHEN key = 'cycles_device' THEN value END), 'auto'),
+                'render_backend',     COALESCE(MAX(CASE WHEN key = 'render_backend' THEN value END), 'celery'),
+                'product_thumbnail_priority', COALESCE(MAX(CASE WHEN key = 'product_thumbnail_priority' THEN value END), '["latest_render","cad_thumbnail"]')
+            ),
+            jsonb_build_object(
+                'concurrency',              COALESCE(MAX(CASE WHEN key = 'celery_worker_concurrency' THEN value END), '8')::int,
+                'max_concurrent_renders',   COALESCE(MAX(CASE WHEN key = 'blender_max_concurrent_renders' THEN value END), '3')::int,
+                'render_stall_timeout_minutes', COALESCE(MAX(CASE WHEN key = 'render_stall_timeout_minutes' THEN value END), '120')::int
+            ),
+            NOW()
+        FROM system_settings
+        WHERE key IN (
+            'thumbnail_renderer', 'blender_engine', 'blender_cycles_samples',
+            'blender_eevee_samples', 'thumbnail_format', 'stl_quality',
+            'blender_smooth_angle', 'cycles_device', 'render_backend',
+            'product_thumbnail_priority', 'celery_worker_concurrency',
+            'blender_max_concurrent_renders', 'render_stall_timeout_minutes'
+        )
+        HAVING COUNT(*) > 0
+    """)
+
+    # Insert default row if no system_settings existed
+    op.execute("""
+        INSERT INTO app_config (render, worker, updated_at)
+        SELECT
+            '{"thumbnail_renderer": "blender", "blender_engine": "cycles",
+              "blender_cycles_samples": 256, "blender_eevee_samples": 64,
+              "thumbnail_format": "jpg", "stl_quality": "low",
+              "blender_smooth_angle": 30, "cycles_device": "auto",
+              "render_backend": "celery",
+              "product_thumbnail_priority": ["latest_render", "cad_thumbnail"]}'::jsonb,
+            '{"concurrency": 8, "max_concurrent_renders": 3, "render_stall_timeout_minutes": 120}'::jsonb,
+            NOW()
+        WHERE NOT EXISTS (SELECT 1 FROM app_config)
+    """)
+
+
+def downgrade():
+    op.drop_table('app_config')