fix(projects): three project-detail page hotfixes

m hit a cluster of three bugs on /projects/{id}/submissions:

1. 500 on /api/projects/{id}/partner-units — DerivationService.AttachedUnit
   scanned derive_unit_roles (text[]) into a plain []string. sqlx returns
   []uint8 for array columns without an adapter. Swap to pq.StringArray
   (same shape as the other array-scanned types in the codebase).

2. 404 on /projects/{id}/submissions — every other project-tab path
   (history, deadlines, team, checklists, …) is registered in handlers.go
   routing all to handleProjectsDetailPage so deep links work, but the
   submissions tab added in t-paliad-230 never got the matching route.
   Result: m navigates to the share-able URL and gets the 404 chrome.
   Add the missing route entry.

3. Create / update project rejected by projekte_client_number_check —
   the CHECK is `client_number IS NULL OR matches '^[0-9]{6}$'`, but the
   form sends empty string "" for an unset field. The Create path passed
   `*input.ClientNumber` raw; the Update path's appendSetSkippable did
   the same. Both now route through a new nullableTrimmed helper that
   coerces empty/whitespace to nil → SQL NULL → constraint accepts.
   matter_number gets the same treatment for symmetry.

Verified the SQL by EXPLAIN against the live DB on the today-filter
hotfix (becf4f0). These three fixes only change Go-side type / nil-
coercion, so no SQL-syntax exposure.

internal/handlers/handlers.go

@@ -466,6 +466,10 @@ func Register(mux *http.ServeMux, client *auth.Client, giteaAPIToken string, svc
 	protected.HandleFunc("GET /projects/{id}/notes", gateOnboarded(handleProjectsDetailPage))
 	protected.HandleFunc("GET /projects/{id}/checklists", gateOnboarded(handleProjectsDetailPage))
 	protected.HandleFunc("GET /projects/{id}/team", gateOnboarded(handleProjectsDetailPage))
+	// t-paliad-230 Schriftsätze tab — same shape as every other tab above.
+	// Without this route the deep-link 404s; the tab still works via
+	// in-page click since it just toggles a panel.
+	protected.HandleFunc("GET /projects/{id}/submissions", gateOnboarded(handleProjectsDetailPage))
 	// t-paliad-177 — standalone Project Timeline / Chart page (Slice 1).
 	// Horizontal SVG renderer mounted client-side; reuses the existing
 	// /api/projects/{id}/timeline JSON endpoint for data.