fix: stub Starknet airdrop fetcher

[prismn]

Summary

• Remove the Starknet zkLend default endpoint from the airdrop fetcher.
• Make the fetcher a no-op stub unless AIRDROP_REWARD_API_ENDPOINT is explicitly configured.
• Update the claimer to await fetch results when needed and skip claims when no proof data is available.
• Rename the Starknet-specific fetcher tests to neutral airdrop fetcher tests.

Validation

• python -m py_compile web_app/contract_tools/airdrop.py web_app/tasks/claim_airdrops.py web_app/tests/test_airdrop_fetcher.py web_app/tests/test_claim_airdrops.py
• poetry run pytest web_app/tests/test_airdrop_fetcher.py web_app/tests/test_claim_airdrops.py
• poetry run pytest web_app/tests still has pre-existing backend failures unrelated to this change in DB/user/vault tests.
• yarn build in quantara/frontend passed after installing frontend dependencies with yarn install --frozen-lockfile.

Closes #56

[YaronZaki]

Clean work @prismn! 🧹 Making the airdrop fetcher a no-op when not configured is graceful and well-architected. Thanks for addressing #56!

[YaronZaki]

Hey @prismn 👋 Heads up: this PR now has merge conflicts because #127 (CORS env vars docs) was merged into main before yours, and both modify docs/environment_variables.md. Could you rebase onto the latest main and resolve the merge in that file? Sorry for the timing on this one — your stub work itself looks clean and we're eager to get it in. Thanks! 🧹