feat: comprehensive project improvements

- Fix 14 Clippy warnings across server and bot-sdk
- Add 67 unit tests (32 bot-sdk, 34 server, 1 doctest)
- Add Prometheus metrics endpoint (/api/metrics)
- Add structured JSON logging (EIFELDC_LOG_FORMAT=json)
- Add release workflow (Docker push + GitHub Release + Tauri builds)
- Add rate limiting middleware (EIFELDC_RATE_LIMIT)
- Add CORS restriction (EIFELDC_CORS_ORIGINS)
- Add session token expiry (EIFELDC_SESSION_TTL)
- Add input validation (username/password/homeserver length limits)
- Add upload size limit (EIFELDC_MAX_UPLOAD_MB)
- Upgrade Tauri client from v1 to v2
- Add session store with SQLite persistence
- Add proper error types and cleanup across all crates
- Format all code with cargo fmt
- Update CI pipeline with fmt, clippy, test, frontend, and Tauri checks
- Add README with full API reference and setup guide

Cargo.toml

@@ -2,6 +2,8 @@
 resolver = "2"
 members = [
     "server",
+    "bot-sdk",
+    "client",
 ]
 
 [workspace.dependencies]
@@ -13,6 +15,6 @@ tokio = { version = "1", features = ["full"] }
 anyhow = "1"
 thiserror = "1"
 tracing = "0.1"
-tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }
 reqwest = { version = "0.12", features = ["json", "multipart"] }
 url = "2"