#116 [High] Add structured log shipping wired to the shared Winston logger

[james2177]

Description: src/common/logger.ts provides the shared Winston logger referenced by issue #54's work to route Soroban/sweeper logging through it consistently, but there's no destination beyond stdout — logs aren't shipped anywhere for aggregation, search, or alerting. Add a structured log-shipping transport (e.g. a Winston transport to a log aggregation service) configurable via environment variables.

Problem Statement & Context: As on-chain write paths, the sweeper, and WS gateway all grow more consequential (per issues #43–#54), stdout-only logging makes production incident investigation dependent on manually pulling container logs — there's no searchable, retained, cross-instance log store, which will only get harder once the Redis pub/sub multi-instance scaling in issue #34 lands.

Scope & Acceptance Criteria:

• Add a configurable Winston transport (shipping to a log sink — confirm target with maintainers, e.g. an ELK/Loki/hosted-logging endpoint) to src/common/logger.ts, gated behind an environment variable so local dev/CI remain stdout-only.
• Ensure shipped logs retain structured fields (request ID from the existing X-Request-Id handling referenced in issue Replace console.log in IntentsSweeperService with the shared winston logger #87, log level, timestamp, service/module name) rather than flattening to plain strings.
• Out of scope: building dashboards or alerting rules on top of the shipped logs — this issue delivers the shipping mechanism only.

Implementation Guidelines:

1. Key files: src/common/logger.ts, src/config/configuration.ts, src/config/env.validation.ts (add and validate the new env var(s) per the pattern issue Add correlation/request-ID propagation through logging and error handling #88 audits for completeness).
2. Ensure the new transport fails safe — if the log-shipping destination is unreachable, the app must not crash or block request handling (buffer/drop with a local warning, don't throw).
3. Redact sensitive fields before shipping, consistent with the redaction audit scope in issue Extend /health to check Soroban RPC reachability #91 (don't ship key material or unredacted transaction details to a third-party log sink).
4. Testing: add unit tests confirming the transport is only registered when its env var is set, and an integration-style test confirming a redacted-fields log entry never contains the sensitive values it should strip.

Definition of Done:

• Log-shipping transport implemented, redaction verified, tested.
• Acceptance criteria met.
• PR passes CI with test output.
• Reviewed and approved.

Resources: src/common/logger.ts, src/config/configuration.ts, src/config/env.validation.ts

Complexity: High (200 points)