Modernise zod idioms to v4

Replace the deprecated v3 string-method forms with the top-level
validator helpers zod 4 prefers:

- z.string().url()      → z.url()      (3 sites)
- z.string().uuid()     → z.uuid()     (5 sites)
- z.string().datetime() → z.iso.datetime() (9 sites)

Also swap the ZodIssueCode.custom compat-shim reference in
demo-bot.server.ts's superRefine for the bare string literal "custom",
which is v4's idiomatic form. No runtime change.

All 147 tests still pass.

The `parsed.error.issues.map(i => ({field, message}))` pattern in the
five journal API route handlers stays as-is — the fields we read are
unchanged in v4, and zod's new error helpers (z.treeifyError,
z.prettifyError) produce different shapes that would change our
public API error contract.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/journal/app/lib/demo-bot.server.ts

@@ -51,14 +51,14 @@ const PersonaSchema = z
     for (const loc of p.locales) {
       if (!p.content.names[loc]) {
         ctx.addIssue({
-          code: z.ZodIssueCode.custom,
+          code: "custom",
           path: ["content", "names", loc],
           message: `missing name pool for declared locale '${loc}'`,
         });
       }
       if (!p.content.descriptions[loc]) {
         ctx.addIssue({
-          code: z.ZodIssueCode.custom,
+          code: "custom",
           path: ["content", "descriptions", loc],
           message: `missing description pool for declared locale '${loc}'`,
         });