data-pipelines: consolidate to four modules for a more substantial collapse

Subject moves into lab; Session references User directly; Scan and
ScanInfo move into session. Twelve tables collapse to four nodes, with
two boundaries (lab→session, session→imaging) each bundling two foreign
keys into one heavier edge.

Author: dimitri-yatsenko

src/explanation/data-pipelines.md

@@ -30,16 +30,18 @@ pipeline can be viewed at either one.
 **Module level — the collapsed view.** Each node is a Python module, which corresponds to a
 database schema. Each edge represents a bundle of dependencies: the foreign key references
 between tables of the two schemas, together with the Python import dependency between their
-modules. A heavier edge carries a larger bundle — below, the `scan → imaging` edge bundles
-two separate foreign keys, and `reference` supplies both `scan` and `imaging`.
+modules. A heavier edge carries a larger bundle — below, the `lab → session` and
+`session → imaging` edges each bundle two separate foreign keys, and `reference` supplies
+both `session` and `imaging`. Twelve tables collapse to four nodes.
 
 ![Pipeline DAG at the module level](../images/pipeline-modules-collapsed.svg)
 
 **Table level — the expanded view.** Each node is a table; the dashed clusters group the
 tables of each module. Each edge is an individual foreign key constraint — the two foreign
-keys that cross the `scan → imaging` boundary (`Scan → ScanQuality` and
+keys that cross the `session → imaging` boundary (`Scan → ScanQuality` and
 `ScanInfo → MotionCorrection`) appear separately here, having collapsed into the single
-bundled edge above.
+bundled edge above; likewise `Subject → Session` and `Session → User` across
+`lab → session`.
 
 ![Pipeline DAG at the table level](../images/pipeline-modules.svg)