Add Anthropic Claude Tokenizer Integration

[ivelin-web]

• Install and import @anthropic-ai/tokenizer.
• Map each Claude model (Haiku, Sonnet, Opus) to countTokens(text) calls.
• Verify context window limits (e.g., Opus ≈ 200 k tokens).
• Add fallback to text.length / 3.8 if the library does not recognize a new Claude model name.
• Update tokenizers/index.ts (or equivalent) to call countTokens whenever tenant === 'claude'.
• Test in a dummy Claude chat interface (or mock DOM) to confirm the count matches server‐side counts.