feat: yield expiry, bulk epochs endpoint, and OpenTelemetry tracing

[Gbangbolaoluwagbemiga]

Summary

Resolves four assigned issues across epoch data management and observability.

#823 — Track unclaimed yield expiry per epoch

• Added migration 029_epoch_expiry.sql: nullable expires_at TIMESTAMPTZ column on epochs
• YIELD_CLAIM_EXPIRY_DAYS env var (optional, default null = never expires) added to config
• recordEpoch() calculates and stores expires_at when the env var is set
• getUserPendingYield() query filters expires_at > NOW() so expired epochs never appear in pending yield

#824 — Add bulk epoch data endpoint

• YieldService.getEpochsBulk(contractId, from, to) returns epochs in the inclusive range
• GET /api/v1/yields/:contractId/epochs/bulk?from=<epoch>&to=<epoch> — no pagination, single response
• Returns HTTP 400 when to - from > 500 or params are invalid; from must be ≤ to
• Each entry includes yieldAmount, totalShares, yieldPerShare, distributedAt

#825 — Add OpenTelemetry tracing to Express middleware

• Added @opentelemetry/sdk-node, @opentelemetry/auto-instrumentations-node, @opentelemetry/api, and @opentelemetry/exporter-trace-otlp-http to dependencies
• src/instrumentation.ts initialises the OTel SDK; exports traces to OTLP when OTEL_EXPORTER_OTLP_ENDPOINT is set, no-ops when absent
• Imported as the first line of src/index.ts so all auto-instrumentation patches apply before any other module loads

#826 — Add trace context propagation to DB queries

• query() in db/index.ts now wraps every SQL call in a db.query child span
• Span attributes: db.statement (first 80 chars of SQL) and db.response.rows
• Span is attached to context.active() so it appears as a child of the current HTTP request span
• Errors are recorded on the span and re-thrown; span always ends in finally

Closes #823
Closes #824
Closes #825
Closes #826

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