security: await audit writes, add per-turn AssistantPrompt audit (#55)

- Auth.js authorize/signOut: await createAuditEntry on every branch so auth
  events land in the audit store before the JWT is minted / session closes.
  Previously these were fire-and-forget and would be dropped under DB load.
- Assistant chat: make appendPromptInjectionGuard async and await its own
  SecurityAlert audit; add auditUserPromptTurn() that records every user
  message turn as an AssistantPrompt entry containing conversationId, length,
  SHA-256 fingerprint, pageContext and whether the injection guard fired.
  Raw prompt text is intentionally not stored — the hash lets a responder
  correlate a chat transcript with a forensic request without the audit
  store accumulating a plain-text corpus of everything users typed.
- Replace bare crypto.* with explicit node:crypto imports.
- Document the retention posture in docs/security-architecture.md §6.

Fixes gitea #55.

apps/web/src/server/auth.ts

@@ -85,7 +85,7 @@ const config = {
           : await authRateLimiter(rateLimitKeys);
         if (!rateLimitResult.allowed) {
           // Audit failed login (rate limited)
-          void createAuditEntry({
+          await createAuditEntry({
             db: prisma,
             entityType: "Auth",
             entityId: email.toLowerCase(),
@@ -109,7 +109,7 @@ const config = {
         if (!user?.passwordHash) {
           await verify(DUMMY_ARGON2_HASH, password).catch(() => false);
           logger.warn({ email, reason: "user_not_found" }, "Failed login attempt");
-          void createAuditEntry({
+          await createAuditEntry({
             db: prisma,
             entityType: "Auth",
             entityId: email.toLowerCase(),
@@ -127,7 +127,7 @@ const config = {
             { email, userId: user.id, reason: "account_deactivated" },
             "Login blocked — account deactivated",
           );
-          void createAuditEntry({
+          await createAuditEntry({
             db: prisma,
             entityType: "Auth",
             entityId: user.id,
@@ -143,7 +143,7 @@ const config = {
         const isValid = await verify(user.passwordHash, password);
         if (!isValid) {
           logger.warn({ email, reason: "invalid_password" }, "Failed login attempt");
-          void createAuditEntry({
+          await createAuditEntry({
             db: prisma,
             entityType: "Auth",
             entityId: user.id,
@@ -176,7 +176,7 @@ const config = {
           const delta = totpInstance.validate({ token: totp, window: 1 });
           if (delta === null) {
             logger.warn({ email, reason: "invalid_totp" }, "Failed MFA verification");
-            void createAuditEntry({
+            await createAuditEntry({
               db: prisma,
               entityType: "Auth",
               entityId: user.id,
@@ -196,7 +196,7 @@ const config = {
           const accepted = await consumeTotpWindow(prisma, user.id);
           if (!accepted) {
             logger.warn({ email, reason: "totp_replay" }, "TOTP replay attack blocked");
-            void createAuditEntry({
+            await createAuditEntry({
               db: prisma,
               entityType: "Auth",
               entityId: user.id,
@@ -230,8 +230,10 @@ const config = {
         });
 
         logger.info({ email, userId: user.id }, "Successful login");
-        // Audit successful login
-        void createAuditEntry({
+        // Audit successful login. Awaited (not fire-and-forget) so the entry
+        // is durable before we return a session — forensic completeness
+        // matters even if it adds a few ms to the login path.
+        await createAuditEntry({
           db: prisma,
           entityType: "Auth",
           entityId: user.id,
@@ -338,7 +340,7 @@ const config = {
         });
       }
 
-      void createAuditEntry({
+      await createAuditEntry({
         db: prisma,
         entityType: "Auth",
         entityId: userId ?? email,