fix: complete remaining 8 optimization items

- HTTP connection pooling: UpstreamClient & WebsiteClient reuse httpx.Client
- Deduplicate decimal_string into shared app/utils/number.py
- Split scheduler transaction: snapshot write → webhook/website sync in separate sessions
- Remove hardcoded 170.106.100.210 migration from database.py
- Reset consecutive_failures on upstream update
- Healthcheck: install curl, replace python -c with curl -f
- Add .dockerignore to reduce build context
- Frontend: add axios-retry with exponential backoff (5xx/network errors only)

backend/app/utils/number.py

@@ -0,0 +1,25 @@
+"""Shared numeric formatting utilities."""
+from __future__ import annotations
+
+from decimal import Decimal, InvalidOperation
+from typing import Any
+
+
+def decimal_string(value: Any) -> str:
+    """Format a numeric value as a clean decimal string.
+
+    - None / empty → ""
+    - Whole numbers → no decimal point (e.g. "5")
+    - Decimals → trailing zeros stripped (e.g. "3.14")
+    - Unparseable → raw str()
+    """
+    if value is None or value == "":
+        return ""
+    try:
+        d = Decimal(str(value))
+    except (InvalidOperation, ValueError):
+        return str(value)
+    n = d.normalize()
+    if n == n.to_integral():
+        return str(n.quantize(Decimal("1")))
+    return format(n, "f")