feat: add structured JSON logging with structlog

[eischideraa-unn]

Description

Adds structured JSON logging throughout the backend using structlog. All existing unstructured logging.getLogger / f-string log calls have been replaced with key=value structured calls. A new web_app/utils/logger.py module centralises logging configuration, wallet masking, and logger instantiation. A request_id middleware ensures every log entry emitted during a request lifecycle is traceable by a unique ID.

Key behaviours:

JSON output in production (ENV_VERSION=PROD), human-readable coloured output in development

Every log entry carries: timestamp, level, logger, request_id, event

wallet_id is automatically masked (GABCDE****WXYZ) in all log entries — Stellar public keys embedded in message strings are also scrubbed via regex

X-Request-Id header is accepted from the client or auto-generated (UUID), bound to the structlog context, and echoed in the response

Related Issue
Closes #81 (obs: Add structured JSON logging throughout backend)

Change Type

• feat — new feature

• fix — bug fix

• docs — documentation

• refactor — code refactoring

• test — adding or updating tests

• ci — CI/CD changes

Testing Done
Unit tests added in web_app/tests/test_structured_logging.py:

TestMaskWalletId — verifies correct masking for normal keys, short strings, empty strings, and 12-char boundary inputs

TestMaskWalletProcessor — verifies wallet_id field masking, in-message Stellar key scrubbing, and safe pass-through for messages with no wallet data

TestConfigureLoggingJson — verifies JSON output is valid and contains required fields (level, timestamp) in production mode

TestGetLogger — verifies get_logger() returns valid logger instances

TestRequestIdMiddleware — verifies X-Request-Id is present on every response, that a client-supplied ID is echoed back unchanged, and that auto-generated IDs are valid UUIDs

All existing test suites remain unmodified and passing.

Screenshots (if UI changes)
N/A — backend logging only, no UI changes.