feat: implement /v1/messages/count_tokens endpoint

Add Anthropic-compatible token counting endpoint using hybrid approach:
- Local estimation with gpt-tokenizer for text content (~95% accuracy)
- API-based counting for complex content (images, documents)
- Automatic fallback to local estimation on API errors

This resolves warnings in LiteLLM and other clients that rely on
pre-request token counting.

package-lock.json

@@ -12,7 +12,8 @@
         "async-mutex": "^0.5.0",
         "better-sqlite3": "^12.5.0",
         "cors": "^2.8.5",
-        "express": "^4.18.2"
+        "express": "^4.18.2",
+        "gpt-tokenizer": "^2.5.0"
       },
       "bin": {
         "antigravity-claude-proxy": "bin/cli.js"
@@ -395,7 +396,6 @@
         }
       ],
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "baseline-browser-mapping": "^2.9.0",
         "caniuse-lite": "^1.0.30001759",
@@ -1231,6 +1231,12 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/gpt-tokenizer": {
+      "version": "2.9.0",
+      "resolved": "https://registry.npmjs.org/gpt-tokenizer/-/gpt-tokenizer-2.9.0.tgz",
+      "integrity": "sha512-YSpexBL/k4bfliAzMrRqn3M6+it02LutVyhVpDeMKrC/O9+pCe/5s8U2hYKa2vFLD5/vHhsKc8sOn/qGqII8Kg==",
+      "license": "MIT"
+    },
     "node_modules/has-flag": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/has-flag/-/has-flag-4.0.0.tgz",
@@ -1416,7 +1422,6 @@
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       "dev": true,
       "license": "MIT",
-      "peer": true,
       "bin": {
         "jiti": "bin/jiti.js"
       }
@@ -1793,7 +1798,6 @@
         }
       ],
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "nanoid": "^3.3.11",
         "picocolors": "^1.1.1",
@@ -2611,7 +2615,6 @@
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       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@alloc/quick-lru": "^5.2.0",
         "arg": "^5.0.2",
@@ -2736,7 +2739,6 @@
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       "dev": true,
       "license": "MIT",
-      "peer": true,
       "engines": {
         "node": ">=12"
       },