feat: add secrets provider architecture

Trait-based secrets management for secure credential handling:
- SecretsProvider trait for pluggable backends
- EnvProvider as default (12-Factor App pattern)
- ProtectedString with zeroize (memory zeroed on drop)
- Redacted Debug impl prevents secret leakage in logs
- S3Credentials struct for future AWS S3 integration
- Config: [secrets] section with provider and clear_env options

Foundation for AWS Secrets Manager, Vault, K8s (v0.4.0+)

Cargo.lock

@@ -1230,6 +1230,7 @@ dependencies = [
  "utoipa-swagger-ui",
  "uuid",
  "wiremock",
+ "zeroize",
 ]
 
 [[package]]
@@ -2955,6 +2956,20 @@ name = "zeroize"
 version = "1.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b97154e67e32c85465826e8bcc1c59429aaaf107c1e4a9e53c8d8ccd5eff88d0"
+dependencies = [
+ "zeroize_derive",
+]
+
+[[package]]
+name = "zeroize_derive"
+version = "1.4.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "85a5b4158499876c763cb03bc4e49185d3cccbabb15b33c627f7884f43db852e"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
 
 [[package]]
 name = "zerotrie"