feat: add version API and solar KPI endpoint (v1.4.0)

New endpoints:
- GET /api/v1/version — returns VERSIONS.json (no auth required)
  For field engineers to check platform version from login page
- GET /api/v1/kpi/solar — returns PR, self-consumption rate,
  equivalent utilization hours, and daily revenue
  Handles station-level vs device-level data deduplication

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/v1/version.py

@@ -0,0 +1,32 @@
+import os
+import json
+from fastapi import APIRouter
+
+router = APIRouter(prefix="/version", tags=["版本信息"])
+
+
+@router.get("")
+async def get_version():
+    """Return platform version information for display on login/dashboard"""
+    # Read VERSIONS.json from project root (2 levels up from backend/)
+    backend_dir = os.path.dirname(os.path.dirname(os.path.dirname(__file__)))
+
+    # Try multiple paths for VERSIONS.json
+    for path in [
+        os.path.join(backend_dir, "VERSIONS.json"),            # standalone
+        os.path.join(backend_dir, "..", "VERSIONS.json"),       # inside core/ subtree
+        os.path.join(backend_dir, "..", "..", "VERSIONS.json"), # customer project root
+    ]:
+        if os.path.exists(path):
+            with open(path, 'r', encoding='utf-8') as f:
+                versions = json.load(f)
+            return versions
+
+    # Fallback: read VERSION file
+    version_file = os.path.join(backend_dir, "VERSION")
+    version = "unknown"
+    if os.path.exists(version_file):
+        with open(version_file, 'r') as f:
+            version = f.read().strip()
+
+    return {"project_version": version, "project": "ems-core"}