feat: add SHA256 model integrity check before joblib.load()

Add verify_model_checksum() that validates model files against a
models/checksums.sha256 manifest before loading. Fails open when
manifest is missing or file not listed (backward compat), raises
HTTP 500 on hash mismatch. Created empty manifest placeholder.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

openspec/changes/code-review-fix/tasks.md

@@ -42,7 +42,7 @@
 ## 5. ML Service Hardening (Python)
 
 - [x] 5.1 `[sonnet]` Replace `error.message` / traceback details with generic `"Internal server error"` in FastAPI exception handlers at lines 640, 778, 1091, 1134, 1199, 1296 of `services/ml/app/main.py`
-- [ ] 5.2 `[opus]` Add SHA256 model integrity check: create `models/checksums.sha256` manifest, verify hash before `joblib.load()` in `services/ml/app/main.py:266`
+- [x] 5.2 `[opus]` Add SHA256 model integrity check: create `models/checksums.sha256` manifest, verify hash before `joblib.load()` in `services/ml/app/main.py:266`
 - [ ] 5.3 `[sonnet]` Add `_model_swap_lock` to prediction reads (not just writes) in `services/ml/app/main.py` for thread-safe model access
 - [ ] 5.4 `[sonnet]` Add date range validation (max 1 year) to `POST /predict/batch` in `services/ml/app/main.py`
 - [ ] 5.5 `[sonnet]` Add candle time-sort validation/auto-sort to `POST /predict` in `services/ml/app/main.py`

services/ml/app/main.py

@@ -4,6 +4,7 @@ FastAPI inference service for candlestick pattern prediction.
 Provides REST API endpoints for model serving, health checks, and prediction.
 """
 
+import hashlib
 import logging
 import os
 import re
@@ -258,6 +259,71 @@ def load_model_from_mlflow(model_name: str, stage: str) -> tuple[Any, Dict[str,
         raise
 
 
+def verify_model_checksum(model_path: Path) -> None:
+    """
+    Verify model file integrity against SHA256 checksum manifest.
+
+    Looks for checksums in models/checksums.sha256. If the manifest exists and
+    contains an entry for the model file, verifies the SHA256 hash matches.
+    Raises HTTPException on mismatch. Logs a warning and continues if the
+    manifest is missing or the file is not listed (fail-open for backward
+    compatibility).
+
+    Args:
+        model_path: Path to the model file to verify.
+
+    Raises:
+        HTTPException: If checksum verification fails (hash mismatch).
+    """
+    manifest_path = Path("models/checksums.sha256")
+    model_path = Path(model_path)
+
+    if not manifest_path.exists():
+        logger.warning("Model checksum manifest not found at %s — skipping integrity check", manifest_path)
+        return
+
+    # Parse manifest: each line is "<sha256hash>  <filename>"
+    checksums: Dict[str, str] = {}
+    try:
+        for line in manifest_path.read_text().splitlines():
+            line = line.strip()
+            if not line or line.startswith("#"):
+                continue
+            parts = line.split(None, 1)
+            if len(parts) == 2:
+                checksums[parts[1].strip()] = parts[0].strip()
+    except Exception as e:
+        logger.warning("Failed to read checksum manifest: %s — skipping integrity check", e)
+        return
+
+    filename = model_path.name
+    if filename not in checksums:
+        logger.warning("No checksum entry for '%s' in manifest — skipping integrity check", filename)
+        return
+
+    # Compute SHA256 of the actual file
+    sha256 = hashlib.sha256()
+    try:
+        with open(model_path, "rb") as f:
+            for chunk in iter(lambda: f.read(8192), b""):
+                sha256.update(chunk)
+    except Exception as e:
+        logger.error("Failed to compute SHA256 for '%s': %s", model_path, e)
+        raise HTTPException(status_code=500, detail="Internal server error")
+
+    actual_hash = sha256.hexdigest()
+    expected_hash = checksums[filename]
+
+    if actual_hash != expected_hash:
+        logger.error(
+            "Model integrity check FAILED for '%s': expected %s, got %s",
+            filename, expected_hash, actual_hash
+        )
+        raise HTTPException(status_code=500, detail="Internal server error")
+
+    logger.info("Model integrity check passed for '%s'", filename)
+
+
 def load_model_from_local(model_path: str) -> tuple[Any, Dict[str, Any]]:
     """
     Load model from local file using joblib.
@@ -278,6 +344,9 @@ def load_model_from_local(model_path: str) -> tuple[Any, Dict[str, Any]]:
     if not model_path.exists():
         raise FileNotFoundError(f"Model file not found: {model_path}")
     
+    # Verify model integrity before loading
+    verify_model_checksum(model_path)
+
     try:
         # Load model with joblib
         model_data = joblib.load(model_path)