refactor(arch-09): introduce BuildMetadataProperties — decouple build provenance from dd.* namespace

[ff-team-sobrado]

Summary

ARCH-09-followup-2 (Task #18) — second concrete refactor from the
ARCH-09 audit. Replaces four scattered @Value injection sites
(two on each of PaymentIntentSucceededHandler and
PaymentIntentFailedHandler) with a single typed
BuildMetadataProperties bean.

Mirrors the InvestmentPaymentProperties shape established here in
PR #6 (and WaitlistProperties on sobrado-site-api) — the canonical
Micronaut idiom for grouped configuration the ARCH-09 audit
established for every future Lambda.

Companion MR on pii-ingestion (pii-ingestion!4) applies the same
refactor to DynamoDbInvestorIbanStore; this PR is the larger of the
pair (two consumers, four @Value sites).

Naming improvement

Pre-refactor, the Lambda read the build version through the dd.*
(Datadog) namespace because Micronaut auto-maps the DD_VERSION env
var to dd.version. That conflated two concerns: the Datadog APM
"version" tag and Sobrado's build provenance.

Post-refactor the YAML binds build.version and build.git-sha to
the underlying env vars explicitly:

```yaml
build:
version: ${DD_VERSION:unknown}
git-sha: ${GIT_SHA:unknown}
```

The env-var contract is preserved byte-for-byte (no infrastructure
change required); only the Java-side namespace decouples from the
Datadog convention. Future: when a dedicated build-provenance source
is wanted (e.g. CI-injected BUILD_VERSION), only the YAML changes
— application code already reads through the typed bean.

Audit-log JSON keys (lambdaVersion, gitSha) preserved verbatim —
no DDB AuditLog row schema change.

Test plan

• ./gradlew test — green locally on feat/arch-09-followup-build-metadata-properties.
• PaymentIntentSucceededHandlerTest — both ctor sites (default
  setUp + degraded-mode test) updated; introduces LAMBDA_VERSION
  / GIT_SHA constants + newBuildMetadata helper.
• PaymentIntentFailedHandlerTest — same shape, mirrored helper.
• CI verify pipeline green (Gradle build + tests).
• CI build-image pipeline green (GraalVM native image still builds
  with the new YAML key + bean shape).

Diff stat

```
.../config/BuildMetadataProperties.java | 74 ++++++++++++ (new)
.../dispatch/handlers/PaymentIntentFailedHandler.java | 11 +++++++----
.../handlers/PaymentIntentSucceededHandler.java | 11 +++++++----
src/main/resources/application.yml | 9 +++++++++
.../handlers/PaymentIntentFailedHandlerTest.java | 18 ++++++++++++++++--
.../handlers/PaymentIntentSucceededHandlerTest.java | 18 ++++++++++++++++--
```

Companion MR

• pii-ingestion: !4 (same shape; 1 consumer).