Add wallet activity-calendar and largest-transfers endpoints

[yosemite01]

Summary

Closes #28, closes #29.

• GET /api/v1/wallets/{address}/activity-calendar — per-UTC-day transaction count for an address, sourced from the ingested payments table. Boundary is UTC midnight via SQLite date(), matching the convention noted in Add /api/v1/wallets/{address}/activity-calendar endpoint #28/Add /api/v1/network/daily-active-accounts time-series endpoint #15.
• GET /api/v1/wallets/{address}/largest-transfers — the address's largest transfers (incoming or outgoing), ranked by USD value via the existing price feed rather than raw asset amount, since comparing a large XLM transfer to a small USDC transfer by raw amount is meaningless (per Add /api/v1/wallets/{address}/largest-transfers endpoint #29's own note). Ranking is computed over the address's most recent 1000 transfers to keep the query and the price-feed conversion pass bounded — documented as a TRANSFER_CANDIDATE_POOL constant, not silently assumed.
• Assets with no price-feed mapping are excluded from the "largest" ranking rather than guessed at, since Add most-active-counterparties aggregation: DEX vs payment address classification #10 (counterparty/DEX classification) hasn't landed yet — this PR doesn't attempt that nice-to-have.
• Added PriceFeedClient::with_provider() so handlers that depend on price conversion can be exercised with an injected provider in tests, instead of making real network calls to CoinGecko.
• Registered both endpoints in the OpenAPI doc (Wallets tag).

Unrelated fixes included

main did not compile at all on a clean checkout. Since that blocks verifying any change, this PR's first commit repairs four small, unrelated pre-existing syntax bugs that were the direct compile blockers:

• src/queue/mod.rs: a stray //! inner-doc-comment block mid-file (invalid placement).
• src/services/realtime_broadcaster.rs, src/shutdown.rs: #[allow(clippy::expect_used)] attributes placed mid method-chain (invalid).
• src/rpc/stellar.rs: a duplicated/dead code block left inside an else branch after a return, leaving a brace unclosed for the rest of the file.

After that fix, the build still fails with ~149 unrelated errors (dependency/API mismatches touching crypto signing, redis, tracing/opentelemetry, HMAC) — filed as #51, intentionally not addressed here since it's a much larger, unrelated cleanup.

Test plan

• Could not run cargo test/cargo clippy locally — blocked by the pre-existing, unrelated errors tracked in main branch fails to compile: ~149 pre-existing errors across unrelated subsystems #51. New code was reviewed carefully against this repo's existing conventions (Database/ApiError/sqlx::FromRow patterns from corridors.rs, account_merges.rs, db/aggregation.rs; price feed batch API from price_feed.rs).
• Once main branch fails to compile: ~149 pre-existing errors across unrelated subsystems #51 is resolved (or in an environment with matching dependency versions), run: cargo test wallets, cargo clippy --lib.
• Manual check: GET /api/v1/wallets/{address}/activity-calendar?days=30 and GET /api/v1/wallets/{address}/largest-transfers?limit=10 against a DB with seeded payments rows.