docs: modernize tutorial schema diagrams with generated dj.Diagram SVGs (#255)

Replace the old mermaid-exported pipeline figures with real dj.Diagram renders
(schema 'imaging'/'ephys'/lab), in the modernized notation and theme-adaptive
(auto light/dark):

- calcium-pipeline.svg, ephys-pipeline.svg, ephys-npy-pipeline.svg regenerated
  from their tutorial schemas (parts, entity clusters, cardinality edges).
- first-pipeline.svg newly rendered; the 01-first-pipeline tutorial's hand-written
  mermaid is replaced with the generated SVG (it now shows the dashed secondary
  Researcher->Session reference correctly).
- Delete the orphaned concepts-table-tiers-diagram.md (referenced nowhere, old
  palette).

Flowchart/state/architecture mermaid diagrams elsewhere are intentionally left as
mermaid — dj.Diagram only renders schemas.

Author: dimitri-yatsenko