feat: add Ethereum chain support

[Ridwannurudeen]

Summary

Adds Ethereum chain support alongside the existing Solana infrastructure, covering the three items in #48:

• ETH portfolio analysis — EthereumPortfolioFetcher with dual-mode support: auto-detects Alchemy endpoints and uses alchemy_getTokenBalances for complete ERC20 coverage in a single RPC call, with graceful fallback to individual balanceOf queries for 10 well-known tokens (USDC, USDT, DAI, WETH, WBTC, LINK, UNI, AAVE, CRV, stETH)
• ETH token metadata — Hardcoded ETH/USDC/USDT/DAI entries with search_token() shortcut matching, backed by existing DexScreener API (already chain-agnostic)
• ETH wallet tokens — Portfolio fetcher resolves token metadata and USD values via TokenMetadataRepo

Also adds Ethereum DeFi protocol implementations for pool discovery:

• Uniswap V3 — Top 100 pools by TVL from The Graph subgraph, fee-based APR calculation across 1d/7d/30d windows, fee tier labeling (0.01%-1%)
• Aave V3 — Active lending reserves with supply APR from on-chain liquidityRate (RAY units), USD-denominated TVL via priceInEth * ETH/USD * supply

Architecture

Follows existing patterns exactly:

• Protocols implement Protocol ABC and register with ProtocolRegistry
• Portfolio fetcher mirrors solana_portfolio.py structure
• New retrieve_ethereum_pools tool added to investor agent toolkit
• Chain.ETHEREUM enum already existed — no type changes needed

New files

• onchain/portfolio/ethereum_portfolio.py
• onchain/pools/ethereum/uniswap_v3_protocol.py
• onchain/pools/ethereum/aave_protocol.py
• onchain/pools/ethereum/test_ethereum_protocols.py

Modified files

• onchain/tokens/metadata.py — ETH hardcoded tokens
• agent/tools.py — retrieve_ethereum_pools tool
• server/fastapi_server.py — register ETH protocols + portfolio fetcher
• main.py — ETH protocol names
• .env.example — ETH_RPC_URL, UNISWAP_V3_SUBGRAPH_URL, AAVE_V3_SUBGRAPH_URL

Key improvements over initial implementation

• Alchemy API support — alchemy_getTokenBalances provides complete ERC20 token discovery without maintaining a hardcoded list; falls back gracefully to individual balanceOf calls for non-Alchemy RPCs
• Configurable subgraph URLs — Both Uniswap V3 and Aave V3 subgraph endpoints are configurable via environment variables with sensible defaults
• Accurate Aave TVL — Converts from token-native units to USD using priceInEth * ETH/USD from DexScreener, rather than raw token counts
• Integration tests — Full test coverage for Uniswap V3 pools, Aave V3 reserves, portfolio fetching, empty wallets, and invalid addresses

Dependencies

• web3>=7.3.0 — already in requirements.txt
• aiohttp — already in requirements.txt

Test plan

• Verify ETH token search returns hardcoded tokens (ETH, USDC, USDT, DAI)
• Verify DexScreener metadata fetch works for ethereum chain
• Test EthereumPortfolioFetcher with a known wallet address
• Test Uniswap V3 subgraph query returns pools with valid APR
• Test Aave V3 subgraph query returns lending reserves with USD TVL
• Verify retrieve_ethereum_pools tool filters by Chain.ETHEREUM
• Verify existing Solana functionality is unaffected
• Test portfolio fallback mode (non-Alchemy RPC)
• Test empty wallet and invalid address edge cases

Closes #48