feat(ledger): wire prometheus metrics, otlp tracing and structured json logs

[tiana-code]

What

Wires generic OSS observability into the ledger service (Epic E-01).

• Metrics: two bounded ledger counters via LedgerMetrics - ledger.transactions.posted (tag type=post|reversal) and ledger.balance.reads, incremented on the success path only (after the write + audit commit; failures throw earlier). Enables the http.server.requests percentile histogram. Exposed at /actuator/prometheus as ledger_transactions_posted_total / ledger_balance_reads_total (Micrometer appends _total, so the counter names deliberately omit it).
• Tracing: Micrometer Tracing bridged to OpenTelemetry with an OTLP exporter; endpoint and sampling are env-overridable per profile (dev 1.0, prod ${TRACING_SAMPLING_PROBABILITY:0.1}, test disabled). traceId/spanId flow into the logging MDC via the bridge.
• Structured logs: logging.structured.format.console=logstash on default/test/prod (one JSON line per event carrying correlation_id and trace ids from MDC); dev keeps the human-readable pattern with %X{correlation_id:-}.
• Security: /actuator/prometheus added to PUBLIC_PATHS (mirrors /actuator/health); exposure stays health,info,prometheus.

Tagging / cardinality

Only the bounded type tag is used. No account id, transaction id, amount, actor, or correlation id is ever a metric tag or label. No money value is emitted as a metric.

Tests

• Unit: LedgerMetricsTest (counter names/tags), poster + balance tests assert increment-on-success and no-increment-on-duplicate.
• Integration (ObservabilityIT, CI only): prometheus reachable without a token and contains the posted counter with value >= 1 after a post; a logged line parses as logstash JSON with @timestamp/level/logger_name/message/correlation_id; the Tracer bean and tracing properties are bound.
• MetricsTestConfig supplies a MeterRegistry to the 5 @DataJpaTest slices that import the poster/balance beans (no actuator autoconfig there).

Gate chain: analyst, architect, critic (GO-WITH-CHANGES, folded in), code-reviewer (caught the _total naming bug, fixed), security-auditor PASS, OSS-boundary PASS. Local gates green; integration tests run on CI.

Closes #51