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/server/runtime-env.ts

@@ -10,6 +10,29 @@ const DISALLOWED_PRODUCTION_SECRETS = new Set([
   "ci-test-secret-minimum-32-chars-xx",
 ]);
 
+// A cryptographically generated secret (openssl rand -base64 32 / -hex 32)
+// has ≥ 32 ASCII characters and high Shannon entropy (≥ 4 bits per char
+// for base64, ≥ 4 for hex). Values below these thresholds are either
+// too short to resist offline brute force of the JWT signature, or are
+// low-entropy strings like "password1234567890123456789012345678" that
+// pass a simple length check but are trivially guessable.
+const MIN_AUTH_SECRET_LENGTH = 32;
+const MIN_AUTH_SECRET_SHANNON_ENTROPY = 3.5;
+
+function shannonEntropy(value: string): number {
+  if (value.length === 0) return 0;
+  const counts = new Map<string, number>();
+  for (const ch of value) {
+    counts.set(ch, (counts.get(ch) ?? 0) + 1);
+  }
+  let entropy = 0;
+  for (const count of counts.values()) {
+    const p = count / value.length;
+    entropy -= p * Math.log2(p);
+  }
+  return entropy;
+}
+
 type RuntimeEnv = Partial<Record<string, string | undefined>>;
 
 function readEnvValue(env: RuntimeEnv, ...names: string[]): string | null {
@@ -46,6 +69,17 @@ export function getRuntimeEnvViolations(env: RuntimeEnv = process.env): string[]
     violations.push(
       "AUTH_SECRET or NEXTAUTH_SECRET must not use a known development placeholder in production.",
     );
+  } else {
+    if (authSecret.length < MIN_AUTH_SECRET_LENGTH) {
+      violations.push(
+        `AUTH_SECRET or NEXTAUTH_SECRET must be at least ${MIN_AUTH_SECRET_LENGTH} characters in production.`,
+      );
+    }
+    if (shannonEntropy(authSecret) < MIN_AUTH_SECRET_SHANNON_ENTROPY) {
+      violations.push(
+        "AUTH_SECRET or NEXTAUTH_SECRET entropy is too low; generate with `openssl rand -base64 32`.",
+      );
+    }
   }
 
   violations.push(...getDevBypassViolations(env));