security: align client password policy with server, enforce AUTH_SECRET length + entropy (#56)

Client-side validators (reset-password, invite-accept, first-admin setup,
user-create modal) previously checked password.length < 8 while every
server-side Zod schema required .min(12). External API consumers (or a
confused browser UI) could get past the client check but fail at the tRPC
boundary — or worse, quietly under-enforce policy compared to what
admins expect.

Fix: introduce PASSWORD_MIN_LENGTH (12) and PASSWORD_MAX_LENGTH (128) in
@capakraken/shared and import them from every pre-submit client validator
and every server Zod schema. Single source of truth; drift becomes a
compile error rather than a security finding.

Also hardens the AUTH_SECRET runtime check: in addition to the existing
placeholder-blacklist, production startup now rejects secrets shorter
than 32 chars OR with Shannon entropy below 3.5 bits/char. That covers
low-entropy-but-long values like "aaaa..." (38 chars, entropy 0) which
would have passed the previous checks.

Documented the rotation process for AUTH_SECRET + POSTGRES_PASSWORD in
docs/security-architecture.md §3.

Verified:
- pnpm test:unit — 396 files / 1922 tests passed
- pnpm --filter @capakraken/web exec tsc --noEmit — clean
- pnpm --filter @capakraken/api exec tsc --noEmit — clean

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/web/src/app/setup/actions.ts

@@ -1,6 +1,11 @@
 "use server";
 import { prisma } from "@capakraken/db";
 import { SystemRole } from "@capakraken/db";
+import {
+  PASSWORD_MAX_LENGTH,
+  PASSWORD_MIN_LENGTH,
+  PASSWORD_POLICY_MESSAGE,
+} from "@capakraken/shared";
 
 export type SetupResult =
   | { success: true }
@@ -13,8 +18,14 @@ export async function createFirstAdmin(formData: {
 }): Promise<SetupResult> {
   // Validate
   if (!formData.name.trim()) return { error: "validation", message: "Name is required." };
-  if (!formData.email.includes("@")) return { error: "validation", message: "Valid email required." };
-  if (formData.password.length < 8) return { error: "validation", message: "Password must be at least 8 characters." };
+  if (!formData.email.includes("@"))
+    return { error: "validation", message: "Valid email required." };
+  if (
+    formData.password.length < PASSWORD_MIN_LENGTH ||
+    formData.password.length > PASSWORD_MAX_LENGTH
+  ) {
+    return { error: "validation", message: PASSWORD_POLICY_MESSAGE };
+  }
 
   // TOCTOU guard — check again inside the action
   const count = await prisma.user.count();