Add comprehensive backend test suite (#339)

* add test suite, mostly covers integration testing, tests are only backend side

* workflow runs the correct script

* workflow runs the correct script

* workflow runs the correct script

* unit tests incoming

* Fix multer silent rejections and error handler info leak

- Revert cb(null, false) to cb(new Error(...)) in auth.ts, collab.ts,
  and files.ts so invalid uploads return an error instead of silently
  dropping the file
- Error handler in app.ts now always returns 500 / "Internal server
  error" instead of forwarding err.message to the client

* Use statusCode consistently for multer errors and error handler

- Error handler in app.ts reads err.statusCode to forward the correct
  HTTP status while keeping the response body generic

server/tests/fixtures/test.gpx

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<gpx version="1.1" creator="TREK Tests" xmlns="http://www.topografix.com/GPX/1/1">
+  <wpt lat="48.8566" lon="2.3522">
+    <name>Eiffel Tower</name>
+    <desc>Paris landmark</desc>
+  </wpt>
+  <wpt lat="48.8606" lon="2.3376">
+    <name>Louvre Museum</name>
+    <desc>Art museum</desc>
+  </wpt>
+</gpx>

server/tests/fixtures/test.pdf

@@ -0,0 +1,21 @@
+%PDF-1.4
+1 0 obj
+<< /Type /Catalog /Pages 2 0 R >>
+endobj
+2 0 obj
+<< /Type /Pages /Kids [3 0 R] /Count 1 >>
+endobj
+3 0 obj
+<< /Type /Page /Parent 2 0 R /MediaBox [0 0 100 100] >>
+endobj
+xref
+0 4
+0000000000 65535 f 
+0000000009 00000 n 
+0000000058 00000 n 
+0000000115 00000 n 
+trailer
+<< /Size 4 /Root 1 0 R >>
+startxref
+190
+%%EOF