security: workbook path allowlist + stronger image polyglot validation (#54)

- dispo workbook imports are pinned to DISPO_IMPORT_DIR (default ./imports):
  tRPC input rejects absolute paths and .. segments, runtime reader
  re-validates containment via path.relative. Closes a path-traversal
  class that reached ExcelJS CVEs through admin/compromised tokens.
- image validator now checks the full 8-byte PNG magic, enforces PNG IEND
  and JPEG EOI trailers, scans the decoded buffer for markup polyglot
  markers (<script, <svg, <iframe, javascript:, onerror=, ...), and
  explicitly rejects SVG. Provider-generated covers (DALL-E, Gemini) run
  through the same validator before persistence — an untrusted upstream
  cannot smuggle a stored-XSS payload past us.
- added image-validation.test.ts and tightened documentation.

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

packages/api/src/router/project-cover.ts

@@ -100,6 +100,18 @@ export const projectCoverProcedures = {
             message: `Gemini error: ${parseGeminiError(err)}`,
           });
         }
+
+        // Provider-generated output is still untrusted — a compromised or
+        // misconfigured upstream could return a polyglot payload. Run the
+        // same magic-byte + trailer + marker check we apply to user uploads
+        // before we persist the data URL to the database.
+        const providerCheck = validateImageDataUrl(coverImageUrl);
+        if (!providerCheck.valid) {
+          throw new TRPCError({
+            code: "INTERNAL_SERVER_ERROR",
+            message: `Provider image rejected by validator: ${providerCheck.reason}`,
+          });
+        }
       } else {
         const dalleClient = createDalleClient(runtimeSettings);
         const model =
@@ -135,6 +147,14 @@ export const projectCoverProcedures = {
         }
 
         coverImageUrl = `data:image/png;base64,${b64}`;
+
+        const providerCheck = validateImageDataUrl(coverImageUrl);
+        if (!providerCheck.valid) {
+          throw new TRPCError({
+            code: "INTERNAL_SERVER_ERROR",
+            message: `Provider image rejected by validator: ${providerCheck.reason}`,
+          });
+        }
       }
 
       await ctx.db.project.update({