fix: resolve config folders API bugs and test infrastructure

- Fix validation error handler to serialize ValueError objects safely
- Add GET /config-folders/{id} endpoint (was missing)
- Fix project overrides API to accept project_id in body instead of query param
- Add flag_modified for SQLAlchemy JSONB change detection
- Fix DELETE endpoint to return 204 status code
- Fix conftest.py to use single SQLite engine per test
- Install aiosqlite dependency
- Fix frontend ToolWorkshopPage tests button names

Config folders tests: 13/13 passing
Docker build tests: 10/10 passing
Readiness probe tests: 13/13 passing

apps/api/src/main.py

@@ -1,3 +1,4 @@
+import json
 import logging
 import os
 
@@ -59,6 +60,32 @@ app.add_middleware(RequestLoggingMiddleware)
 app.add_middleware(ExceptionLoggingMiddleware)
 
 
+def _sanitize_validation_errors(errors):
+    """Convert validation errors to JSON-safe format."""
+    sanitized = []
+    for error in errors:
+        safe_error = {
+            "type": error.get("type"),
+            "loc": error.get("loc"),
+            "msg": error.get("msg"),
+            "input": str(error.get("input")) if error.get("input") is not None else None,
+        }
+        # Convert ctx to safe format
+        ctx = error.get("ctx")
+        if ctx:
+            safe_ctx = {}
+            for key, value in ctx.items():
+                if isinstance(value, Exception):
+                    safe_ctx[key] = str(value)
+                elif isinstance(value, (str, int, float, bool, type(None))):
+                    safe_ctx[key] = value
+                else:
+                    safe_ctx[key] = str(value)
+            safe_error["ctx"] = safe_ctx
+        sanitized.append(safe_error)
+    return sanitized
+
+
 @app.exception_handler(RequestValidationError)
 async def validation_exception_handler(request: Request, exc: RequestValidationError):
     """Log validation errors and return detailed response."""
@@ -69,9 +96,10 @@ async def validation_exception_handler(request: Request, exc: RequestValidationE
         request.url.path,
         errors,
     )
+    safe_errors = _sanitize_validation_errors(errors)
     return JSONResponse(
         status_code=422,
-        content={"detail": errors},
+        content={"detail": safe_errors},
     )