feat: testnet guard csv export file validation deployment meta

[AbayomiCoded]

Implement testnet config guardrails, streaming CSV export, file validation, and deployment metadata

Overview

This PR adds critical infrastructure hardening including testnet configuration guardrails, streaming CSV export for large datasets, file upload validation, and deployment metadata exposure.

Changes

Testnet Mode Config Guardrails (#555)

• Centralize testnet configuration (network passphrase, explorer URLs)
• Prevent mainnet values in testnet environment with fast-fail startup validation
• Provide clear error messages identifying the invalid variable

Streaming CSV Export for Campaigns (#554)

• Implement streaming CSV export for campaigns and claims to prevent memory spikes
• Use streaming response with basic authentication and role checks
• Add load tests validating non-buffering behavior

Evidence Upload Sessions File Validation (#553)

• Enforce mime detection, size limits, and allowed extensions for evidence uploads
• Reject unknown or unsafe types with clear error messages
• Add tests covering allowed and disallowed cases

Deployment Metadata (#550)

• Expose deployment metadata (git sha, environment, build timestamp) via /health or /meta endpoint
• Provide safe defaults when metadata is missing
• Add E2E tests validating response shape

Files Added

• Streaming CSV export implementation
• File validation middleware for uploads
• Deployment metadata endpoint

Files Modified

• Network configuration validation
• Evidence upload handler
• Health/meta endpoint

Closes #555
Closes #554
Closes #553
Closes #550

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