feat: tenant AI chat agent with function calling

Actionable AI assistant that uses per-tenant Azure OpenAI credentials
to execute natural language commands against the render pipeline.

Backend:
- ChatMessage model + migration (session-based conversations)
- Chat service with 10 OpenAI function-calling tools:
  list_orders, search_products, create_order, dispatch_renders,
  get_order_status, set_material_override, set_render_overrides,
  get_render_stats, check_materials, query_database
- All tools tenant-scoped (queries filtered by tenant_id)
- Write operations use httpx to call backend API internally
- Chat API: POST /chat/messages, GET /chat/sessions, DELETE session
- Conversation history preserved in DB (last 50 messages per session)

Frontend:
- Slide-out ChatPanel (right side, w-96, animated)
- User/assistant message styling with avatars and timestamps
- Session management (new chat, session history, delete)
- Typing indicator while waiting for AI response
- Floating chat button in bottom-right corner
- Error state for unconfigured AI tenants

Example: "Render all Kugellager products as WebP at 1024x1024"
→ Agent calls search_products + create_order + dispatch_renders

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/alembic/versions/69964e910545_add_chat_messages_table.py

@@ -0,0 +1,48 @@
+"""add chat_messages table
+
+Revision ID: 69964e910545
+Revises: f5906aaf75af
+Create Date: 2026-03-15 11:38:41.189160
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '69964e910545'
+down_revision: Union[str, None] = 'f5906aaf75af'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('chat_messages',
+    sa.Column('id', sa.UUID(), nullable=False),
+    sa.Column('tenant_id', sa.UUID(), nullable=True),
+    sa.Column('user_id', sa.UUID(), nullable=True),
+    sa.Column('session_id', sa.UUID(), nullable=False),
+    sa.Column('role', sa.String(length=20), nullable=False),
+    sa.Column('content', sa.Text(), nullable=False),
+    sa.Column('context_type', sa.String(length=50), nullable=True),
+    sa.Column('context_id', sa.UUID(), nullable=True),
+    sa.Column('token_count', sa.Integer(), nullable=True),
+    sa.Column('created_at', sa.DateTime(), nullable=False),
+    sa.ForeignKeyConstraint(['tenant_id'], ['tenants.id'], ondelete='CASCADE'),
+    sa.ForeignKeyConstraint(['user_id'], ['users.id'], ondelete='SET NULL'),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_chat_messages_session_id'), 'chat_messages', ['session_id'], unique=False)
+    op.create_index(op.f('ix_chat_messages_tenant_id'), 'chat_messages', ['tenant_id'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_chat_messages_tenant_id'), table_name='chat_messages')
+    op.drop_index(op.f('ix_chat_messages_session_id'), table_name='chat_messages')
+    op.drop_table('chat_messages')
+    # ### end Alembic commands ###