fix(buzz-db): return an empty page from get_channel_window when limit is 0

[hasky00]

Summary

get_channel_window in buzz-db can violate its own documented pagination
contract when called with limit == 0.

ChannelWindow::next_cursor is documented as Some iff has_more
(crates/buzz-db/src/thread.rs). The function derives has_more from a
limit + 1 probe row before truncation, but derives next_cursor from the
truncated row set:

q = q.bind(limit as i64 + 1); // limit == 0 → binds 1, fetches the probe row
let mut db_rows = q.fetch_all(pool).await?;
let has_more = db_rows.len() > limit as usize; // 1 > 0 → true
db_rows.truncate(limit as usize);              // → 0 rows
let next_cursor = if has_more {
    match db_rows.last() { Some(row) => …, None => None } // empty → None
} else { None };

With limit == 0 and at least one matching row, this returns
has_more = true and next_cursor = None, breaking the invariant. A
caller that trusts the contract (window.next_cursor.expect("has_more implies next_cursor") — exactly what the module's own pagination test does) panics,
and a keyset-pagination loop would spin forever returning empty pages.

The sole current production caller clamps the limit with .max(1), so this
isn't reachable from the HTTP surface today — but it's a latent contract
violation in a public Database API that any future caller can trip.

Fix

Guard limit == 0 at the top of get_channel_window and return an empty,
exhausted page (rows: [], has_more: false, next_cursor: None). This
honors the invariant and also skips a pointless database round-trip for a
zero-row request.

Test

Adds channel_window_zero_limit_reports_empty_exhausted_page, mirroring the
existing channel_window_* Postgres tests: it inserts rows, requests a
zero-limit window, and asserts the page is empty, has_more is false, and
next_cursor is None. (Marked #[ignore = "requires Postgres"] like its
siblings.)

Verification

• cargo fmt -p buzz-db -- --check — clean
• cargo check -p buzz-db --tests — clean
• cargo clippy -p buzz-db --tests — clean

The new test requires Postgres and follows the repo's existing
#[ignore = "requires Postgres"] convention.

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