fix: critical security hardening — tenant isolation, CORS, error leaking, input validation

1. Tenant isolation bypass (CRITICAL): TenantResolver now verifies user
   has access to X-Tenant-ID via user_tenants lookup before setting context.
   Added VerifyAccess method to TenantLookup interface and TenantService.

2. Consolidated tenant resolution: Removed duplicate resolveTenant() from
   helpers.go and tenant resolution from auth middleware. TenantResolver is
   now the single source of truth. Deadlines and AI handlers use
   auth.TenantFromContext() instead of direct DB queries.

3. CalDAV credential masking: tenant settings responses now mask CalDAV
   passwords with "********" via maskSettingsPassword helper. Applied to
   GetTenant, ListTenants, and UpdateSettings responses.

4. CORS + security headers: New middleware/security.go with CORS
   (restricted to FRONTEND_ORIGIN) and security headers (X-Frame-Options,
   X-Content-Type-Options, HSTS, Referrer-Policy, X-XSS-Protection).

5. Internal error leaking: All writeError(w, 500, err.Error()) replaced
   with internalError() that logs via slog and returns generic "internal
   error" to client. Same for jsonError in tenant handler.

6. Input validation: Max length on title (500), description (10000),
   case_number (100), search (200). Pagination clamped to max 100.
   Content-Disposition filename sanitized against header injection.

Regression test added for tenant access denial (403 on unauthorized
X-Tenant-ID). All existing tests pass, go vet clean.

backend/internal/handlers/dashboard.go

@@ -24,7 +24,7 @@ func (h *DashboardHandler) Get(w http.ResponseWriter, r *http.Request) {
 
 	data, err := h.svc.Get(r.Context(), tenantID)
 	if err != nil {
-		writeError(w, http.StatusInternalServerError, err.Error())
+		internalError(w, "failed to load dashboard", err)
 		return
 	}