Devtoberfest Signups report: readable week axis, total KPI, region breakdown (#2047)

[jung-thomas]

Closes #2047.

The Devtoberfest Signups Analytical List Page (/admin-ui/#/devtoberfestSignups) opened chart-first with the chart X-axis bound to the internal integer weekIndex — meaningless to users. This addresses all three asks in the issue.

What changed

• Readable week on the chart axis. Added a real, groupable weekMonday : Date column so the chart groups on it and the axis shows the week's Monday date (e.g. Sep 7, 2026) instead of weekIndex. No single date-add expression is portable across HANA and SQLite, so weekMonday is supplied per dialect via the CAP-documented pattern: db/sqlite/native.cds (strftime) + db/hana/native.cds (ADD_DAYS), wired through cds.requires.db.[profile].model. The analytics view is split into a JOIN-bearing facts view + a JOIN-free public projection because the compiler refuses to extend a view containing a JOIN.
• Overall total registrations. Added a @UI.KPI header card (SUM(signups), unfiltered grand total). The AnalyticalTable also renders its own grand-total row for the measure.
• Region breakdown. Added a By Region SelectionPresentationVariant (chart grouped by region), selectable via the page's variant management. Every groupable dimension (region / role / edition) is also reachable through the chart's built-in dimension drill-down.

The read-time enrichment now derives weekLabel/cumulativeSignups from the real weekMonday when present, falling back to weekIndex — so existing weekIndex-grouped reads keep working. The weekLabel 'YYYY-Www' string stays a read-time field (no portable ISO-week SQL function).

Verification

• cds build --production: HANA hdbview uses ADD_DAYS; the precompiled gen/srv/csn.json bakes the HANA weekMonday calc (added db/hana to the nodejs build task's model list).
• SQLite $apply groupby(weekMonday) returns readable Monday dates + correct enrichment.
• Production-profile model resolution loads cleanly (no duplicate-definition/re-merge crash).
• Unit suite: 8990 passed, 0 failed. New coverage for the real weekMonday column (unit + hybrid-HANA) and the weekMonday-keyed enrichment path.

Notes / reviewer attention

• Dev/test cost: specifying requires.db.model defeats cds's compiled-model fast path, so a cold cds.connect.to('db') in dev/tests recompiles the model (~500ms vs ~120ms). This is dev/test-only — production uses the precompiled csn. It exposed a latent 30ms timing race in test/unit/rebuild-trigger.test.js, fixed there by mocking resolveTenantSettings so the dispatch unit does no real DB work (proper isolation).
• Please verify on DEV after deploy (I can't render Fiori Elements here): the chart axis shows dates, the Total Registrations KPI card renders, the By Region variant works, and the srv boots cleanly against HANA. Rollback = revert this PR.
• The weekMonday HANA column materialization is a schema change — deploy is a full build (cds build --production + MTA), not --skip-build.