Add Prometheus metrics endpoint

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This pull request introduces Prometheus metrics support to the backend service, enabling improved observability and monitoring. It adds the prom-client dependency, implements metrics middleware and a /metrics endpoint protected by an internal API key, and provides comprehensive tests for the new metrics functionality. Several related dependencies are also added or updated to support this feature.

Prometheus Metrics Integration

• Added the prom-client package and its dependency tdigest to package.json and package-lock.json to enable Prometheus metrics collection and exposition. ([[1]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-495707834ca4b862f9acdfbac70d279023d2c059da13db59594e61ed3354fed5R43), [[2]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR26), [[3]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR11018-R11030), [[4]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR12126-R12134), [[5]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR4493-R4498))
• Implemented metricsMiddleware and metricsHandler in middleware/metrics.js, and registered them in the Express app to collect and expose metrics at /metrics, protected by an internal API key via requireApiKey. ([[1]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-8455472d7e00dd51813de7e63e5b214382a247990f84bf3fbed9fcc3af24ad6fR26-R32), [[2]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-8455472d7e00dd51813de7e63e5b214382a247990f84bf3fbed9fcc3af24ad6fR122), [[3]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-8455472d7e00dd51813de7e63e5b214382a247990f84bf3fbed9fcc3af24ad6fR166-R167))
• Added a comprehensive test suite in metrics.test.ts to verify metrics collection, endpoint protection, and correct label usage. ([backend/src/__tests__/metrics.test.tsR1-R90](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-62459ca54948fc85c1ff2196b73270e0b8df67254649eb252ca0e714d93ca6baR1-R90))

Dependency and Package Metadata Updates

• Marked many dependencies as peer dependencies in package-lock.json to improve dependency management and compatibility. ([[1]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR137), [[2]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR2495), [[3]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR2517), [[4]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR2530), [[5]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR2950), [[6]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR2967), [[7]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR2985), [[8]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR3098), [[9]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR3656), [[10]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR3838), [[11]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR4088), [[12]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR4566), [[13]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR5721), [[14]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR5778), [[15]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR6201), [[16]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR10754), [[17]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR10946), [[18]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR12204), [[19]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR12363), [[20]](https://github.com/LabsCrypt/remitlend/pull/923/files#diff-f89763fc11c1bc48ac3c7ce8a148fbbbc5c2062393ee78664a70273d714daf5cR12608))

These changes collectively provide robust metrics support for the backend, allowing for real-time monitoring and integration with Prometheus-compatible observability stacks.

Closes #860