docs: enhance developer onboarding guide (#308)

[devSoniia]

Summary

Closes AnnabelJoe#308

Comprehensive update to docs/ONBOARDING.md to reduce time-to-first-contribution for new developers.

Changes

• Architecture diagram — ASCII flow from smart meter → API → Stellar → verifier, plus a key data flows table mapping routes to source files
• Prerequisites — added Docker, pinned Rust version (1.78+)
• Setup — added Docker Compose option (Option A) alongside local dev server
• Environment variables — expanded with Redis (idempotency/cache) and observability vars (Logtail, OTEL)
• Running tests — commands for all packages: web unit (vitest), web e2e (Playwright), contracts (cargo test), shared packages
• Troubleshooting — added entries for: idempotency key not working, OpenTelemetry traces missing, Docker port conflicts, inactive meter, missing env vars
• Project structure — expanded with src/ subdirectory breakdown

[github-actions]

🔍 Vercel Preview Deployment

URL: Learn More: https://err.sh/vercel/no-credentials-found

Uses Stellar testnet contract addresses.

[github-actions]

✅ cargo audit

�[1m�[32m    Updating�[0m crates.io index
�[1m�[32m     Locking�[0m 188 packages to latest compatible versions
�[1m�[36m      Adding�[0m arbitrary v1.3.2 �[1m�[33m(available: v1.4.2)�[0m
�[1m�[36m      Adding�[0m crypto-common v0.1.6 �[1m�[33m(available: v0.1.7)�[0m
�[1m�[36m      Adding�[0m darling v0.23.0 �[1m�[31m(requires Rust 1.88.0)�[0m
�[1m�[36m      Adding�[0m darling_core v0.23.0 �[1m�[31m(requires Rust 1.88.0)�[0m
�[1m�[36m      Adding�[0m darling_macro v0.23.0 �[1m�[31m(requires Rust 1.88.0)�[0m
�[1m�[36m      Adding�[0m derive_arbitrary v1.3.2 �[1m�[33m(available: v1.4.2)�[0m
�[1m�[36m      Adding�[0m serde_with v3.20.0 �[1m�[31m(requires Rust 1.88)�[0m
�[1m�[36m      Adding�[0m serde_with_macros v3.20.0 �[1m�[31m(requires Rust 1.88)�[0m
�[1m�[36m      Adding�[0m soroban-builtin-sdk-macros v23.0.1 �[1m�[33m(available: v23.0.2)�[0m
�[1m�[36m      Adding�[0m soroban-env-common v23.0.1 �[1m�[33m(available: v23.0.2)�[0m
�[1m�[36m      Adding�[0m soroban-env-guest v23.0.1 �[1m�[33m(available: v23.0.2)�[0m
�[1m�[36m      Adding�[0m soroban-env-host v23.0.1 �[1m�[33m(available: v23.0.2)�[0m
�[1m�[36m      Adding�[0m soroban-env-macros v23.0.1 �[1m�[33m(available: v23.0.2)�[0m
�[1m�[36m      Adding�[0m soroban-sdk v23.5.3 �[1m�[33m(available: v25.3.0)�[0m
�[1m�[36m      Adding�[0m time v0.3.47 �[1m�[31m(requires Rust 1.88.0)�[0m
�[1m�[36m      Adding�[0m time-core v0.1.8 �[1m�[31m(requires Rust 1.88.0)�[0m
�[1m�[36m      Adding�[0m time-macros v0.2.27 �[1m�[31m(requires Rust 1.88.0)�[0m
�[0m�[0m�[1m�[32m    Fetching�[0m advisory database from `https://github.com/RustSec/advisory-db.git`
�[0m�[0m�[1m�[32m      Loaded�[0m 1098 security advisories (from /home/runner/.cargo/advisory-db)
�[0m�[0m�[1m�[32m    Updating�[0m crates.io index
�[0m�[0m�[1m�[32m    Scanning�[0m Cargo.lock for vulnerabilities (192 crate dependencies)
�[0m�[0m�[1m�[33mCrate:    �[0m derivative
�[0m�[0m�[1m�[33mVersion:  �[0m 2.2.0
�[0m�[0m�[1m�[33mWarning:  �[0m unmaintained
�[0m�[0m�[1m�[33mTitle:    �[0m `derivative` is unmaintained; consider using an alternative
�[0m�[0m�[1m�[33mDate:     �[0m 2024-06-26
�[0m�[0m�[1m�[33mID:       �[0m RUSTSEC-2024-0388
�[0m�[0m�[1m�[33mURL:      �[0m https://rustsec.org/advisories/RUSTSEC-2024-0388
�[0m�[0m�[1m�[33mDependency tree:
�[0mderivative 2.2.0
├── ark-poly 0.4.2
│   └── ark-ec 0.4.2
│       ├── soroban-env-host 23.0.1
│       │   ├── soroban-sdk 23.5.3
│       │   │   ├── multisig-admin 1.0.0
│       │   │   ├── energy-token 1.0.0
│       │   │   ├── community-governance 1.0.0
│       │   │   └── audit-registry 1.0.0
│       │   └── soroban-ledger-snapshot 23.5.3
│       │       └── soroban-sdk 23.5.3
│       └── ark-bls12-381 0.4.0
│           └── soroban-env-host 23.0.1
├── ark-ff 0.4.2
│   ├── soroban-env-host 23.0.1
│   ├── ark-poly 0.4.2
│   ├── ark-ec 0.4.2
│   └── ark-bls12-381 0.4.0
└── ark-ec 0.4.2

�[0m�[0m�[1m�[33mCrate:    �[0m paste
�[0m�[0m�[1m�[33mVersion:  �[0m 1.0.15
�[0m�[0m�[1m�[33mWarning:  �[0m unmaintained
�[0m�[0m�[1m�[33mTitle:    �[0m paste - no longer maintained
�[0m�[0m�[1m�[33mDate:     �[0m 2024-10-07
�[0m�[0m�[1m�[33mID:       �[0m RUSTSEC-2024-0436
�[0m�[0m�[1m�[33mURL:      �[0m https://rustsec.org/advisories/RUSTSEC-2024-0436
�[0m�[0m�[1m�[33mDependency tree:
�[0mpaste 1.0.15
├── wasmi_core 0.13.0
│   └── soroban-wasmi 0.31.1-soroban.20.0.1
│       ├── soroban-env-host 23.0.1
│       │   ├── soroban-sdk 23.5.3
│       │   │   ├── multisig-admin 1.0.0
│       │   │   ├── energy-token 1.0.0
│       │   │   ├── community-governance 1.0.0
│       │   │   └── audit-registry 1.0.0
│       │   └── soroban-ledger-snapshot 23.5.3
│       │       └── soroban-sdk 23.5.3
│       └── soroban-env-common 23.0.1
│           ├── soroban-sdk-macros 23.5.3
│           │   └── soroban-sdk 23.5.3
│           ├── soroban-ledger-snapshot 23.5.3
│           ├── soroban-env-host 23.0.1
│           └── soroban-env-guest 23.0.1
│               └── soroban-sdk 23.5.3
└── ark-ff 0.4.2
    ├── soroban-env-host 23.0.1
    ├── ark-poly 0.4.2
    │   └── ark-ec 0.4.2
    │       ├── soroban-env-host 23.0.1
    │       └── ark-bls12-381 0.4.0
    │           └── soroban-env-host 23.0.1
    ├── ark-ec 0.4.2
    └── ark-bls12-381 0.4.0

�[0m�[0m�[1m�[33mwarning:�[0m 2 allowed warnings found