(stacked) v2: flatten api/ to repo root

[stegaBOB]

Stacked on top of #125. Pure restructure -- no behaviour change.

Layout

api/ subdirectory collapsed back to repo root, then the remaining files regrouped:

src/
├── api/          ← routes, handlers/, responses
├── build/        ← verify worker + log writer
├── onchain/      ← RPC + Otter PDA reads
├── config.rs
├── db.rs
├── errors.rs
├── main.rs
├── state.rs
├── sweep.rs
└── types.rs      (was validation.rs)

7 top-level files + 3 subdirs, down from 13 + 2 after the bare flatten.

What moved where

• handlers/, responses.rs, routes.rs → api/
• build.rs (worker) + logs.rs (log writer) → build/ (as mod.rs + logs.rs)
• onchain/program_authority_retriever.rs → onchain/state.rs (and program_hash_retriver.rs, a 5-line wrapper, folded in here)
• onchain/program_metadata_retriever.rs → onchain/otter.rs (it's specifically about the Otter Verify PDA, not generic chain reads)
• validation.rs → types.rs (it's the newtype module -- Address, WebhookUrl -- not really a validation module)

Inlined / deleted

• misc.rs split: build_repository_url moves to api/responses.rs (its callers are response builders), extract_hash_with_prefix moves into build/mod.rs as a private fn (only the build worker parses solana-verify stdout).
• validate_pubkey / validate_http_url were only called by the corresponding newtype's FromStr; inlined.
• validate_search had one caller; moved into that handler.