feat: RainCalculator with timezone-aware day mapping and month total

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/fixtures/getmeasure-dst.json

@@ -0,0 +1,20 @@
+{
+    "body": {
+        "1792621800": [
+            1
+        ],
+        "1792708200": [
+            2
+        ],
+        "1792794600": [
+            3
+        ],
+        "1792884600": [
+            4
+        ],
+        "1792971000": [
+            9
+        ]
+    },
+    "status": "ok"
+}

tests/fixtures/getmeasure-month.json

@@ -0,0 +1,20 @@
+{
+    "body": {
+        "1790558220": [
+            1
+        ],
+        "1790644620": [
+            2
+        ],
+        "1790731020": [
+            3
+        ],
+        "1790817420": [
+            4
+        ],
+        "1790903820": [
+            5
+        ]
+    },
+    "status": "ok"
+}

tests/fixtures/getmeasure.json

@@ -0,0 +1,40 @@
+{
+    "body": {
+        "1788225420": [
+            4.3
+        ],
+        "1788311820": [
+            0
+        ],
+        "1788398220": [
+            3.2
+        ],
+        "1788484620": [
+            5.1
+        ],
+        "1788571020": [
+            0
+        ],
+        "1788657420": [
+            0
+        ],
+        "1788743820": [
+            15.5
+        ],
+        "1788830220": [
+            1.2
+        ],
+        "1788916620": [
+            6.8
+        ],
+        "1789003020": [
+            0
+        ],
+        "1789089420": [
+            2.4
+        ]
+    },
+    "status": "ok",
+    "time_exec": 0.02,
+    "time_server": 1789129925
+}