feat(geschichte): bound title and JOURNEY intro at all three layers

Title: requireTitle() turns the VARCHAR(255) DB bound into a friendly 400
(GESCHICHTE_TITLE_TOO_LONG). JOURNEY intro: MAX_INTRO_LENGTH = 4000 in
bodyForType() plus a V75 CHECK as atomic backstop (defensive clamp first,
STORY bodies exempt) — GESCHICHTE_INTRO_TOO_LONG. Both codes wired through
ErrorCode.java, errors.ts, getErrorMessage and de/en/es. DB-layer boundary
pins added: exactly-2000 note insert (V74 CHECK) and 4000/4001 intro
inserts against real Postgres. Docs: error-code lists, db puml diagrams.

The matching maxlength attributes land with the component commits.

Review round 3: Markus (b), Nora (1), Sara (DB 2000 boundary).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/architecture/db/db-orm.puml

@@ -357,7 +357,7 @@ package "Supporting" {
     id : UUID <<PK>>
     --
     title : VARCHAR(255) NOT NULL
-    body : TEXT
+    body : TEXT CHECK (JOURNEY: length <= 4000)
     status : VARCHAR(32) NOT NULL
     type : VARCHAR(32) NOT NULL
     author_id : UUID <<FK>>