💫 Add pure Rust Databricks SDK, replace Python auth interop

[renardeinside]

Summary

• Introduces crates/databricks_sdk — a standalone pure Rust crate for Databricks authentication and API access, replacing the Python SDK dependency for auth operations
• Token acquisition via databricks auth token --profile <name> CLI with automatic caching and expiry-aware refresh (40s staleness buffer)
• Removes get_token(), validate_credentials(), and get_forwarded_user_header() from Python interop, eliminating the databricks-sdk Python dependency for dev server auth
• Simplifies TokenManager in proxy.rs to delegate to DatabricksClient instead of manual Instant-based refresh tracking

Test plan

• just check passes (cargo check, clippy, fmt, ruff, prettier, type check)
• databricks auth login --profile DEFAULT then apx dev start — verify token acquired via CLI
• API proxy forwards auth headers correctly (check /api/* requests)
• Let token expire (~1 hour) — verify automatic re-acquisition

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