refactor: Major overhaul with error handling, resilience patterns, and observability

- Add library configuration to support both lib and binary targets
- Implement unified error handling with NCBError throughout the codebase
- Add circuit breaker pattern for external API calls (Voicevox, GCP TTS)
- Introduce comprehensive performance metrics and monitoring
- Add cache persistence with disk storage support
- Implement retry mechanism with exponential backoff
- Add configuration file support (config.toml) with env var fallback
- Enhance logging with structured tracing (debug, warn, error levels)
- Add extensive unit tests for cache, metrics, and circuit breaker
- Update base64 decoding to use modern API
- Improve API error handling for Voicevox and GCP TTS clients

Breaking changes:
- Function signatures now return Result<T, NCBError> instead of panicking
- Cache key structure modified with serialization support

Cargo.toml

@@ -3,6 +3,14 @@ name = "ncb-tts-r2"
 version = "1.11.2"
 edition = "2021"
 
+[lib]
+name = "ncb_tts_r2"
+path = "src/lib.rs"
+
+[[bin]]
+name = "ncb-tts-r2"
+path = "src/main.rs"
+
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
@@ -13,10 +21,15 @@ gcp_auth = "0.5.0"
 reqwest = { version = "0.12.9", features = ["json"] }
 base64 = "0.22.1"
 async-trait = "0.1.57"
-redis = "0.29.2"
+redis = { version = "0.29.2", features = ["aio", "tokio-comp"] }
+bb8 = "0.8"
+bb8-redis = "0.16"
+thiserror = "1.0"
 regex = "1"
 tracing-subscriber = "0.3.19"
 lru = "0.13.0"
+once_cell = "1.19"
+bincode = "1.3"
 tracing = "0.1.41"
 opentelemetry_sdk = { version = "0.29.0", features = ["trace"] }
 opentelemetry = "0.29.1"
@@ -61,3 +74,9 @@ features = [
 [dependencies.tokio]
 version = "1.0"
 features = ["macros", "rt-multi-thread"]
+
+[dev-dependencies]
+tokio-test = "0.4"
+mockall = "0.12"
+tempfile = "3.8"
+serial_test = "3.0"