test(relay): skip 7 PG-dependent media tests when BUZZ_TEST_DATABASE_URL unset

## Problem

`cargo test -p buzz-relay` on a contributor machine without a local Postgres reports **10 failures**, 7 of which are `pool timed out while waiting for an open connection` errors in `crates/buzz-relay/src/api/media.rs::tests`. They are not real regressions; the tests need Postgres but are marked as plain `#[tokio::test]`, so they run unconditionally.

The repo already has the established `#[ignore = "requires a live Postgres: set BUZZ_TEST_DATABASE_URL"]` pattern in `crates/buzz-search/tests/fts_integration.rs`, where live-PG tests are excluded from the default `cargo test` run but re-enabled via `cargo test -- --include-ignored` plus `BUZZ_TEST_DATABASE_URL=postgres://…`.

This makes the default developer loop produce a wall of "failures" that are really environmental — new contributors can't tell whether they broke something, and CI/suite signal gets diluted.

## Failing tests (all `crates/buzz-relay/src/api/media.rs::tests`)

- `media_get_auth_flag_on_accepts_range_header_only_after_auth`
- `media_get_auth_flag_on_rejects_unauthenticated_get_and_head_before_sidecar_gate`
- `media_get_auth_flag_on_rejects_upload_verb_wrong_server_and_wrong_x`
- `media_get_auth_flag_on_valid_server_scoped_token_reaches_sidecar_gate`
- `media_get_auth_flag_off_allows_unauthenticated_read_until_sidecar_gate`
- `upload_concurrency_limit_is_scoped_by_community`
- `upload_rate_limiter_is scoped_by_community`

## Proposed fix

Apply the same `#[ignore]` annotation so `cargo test -p buzz-relay` reports `0 failed; 42 ignored` on a clean checkout (up from `0 ignored` today). Tests remain runnable via `cargo test -p buzz-relay -- --include-ignored` when `BUZZ_TEST_DATABASE_URL` is set.

Out of scope in this issue:

- The 2 remaining pre-existing fails (`api::admin::tests::feedback_attachment_rejects_unknown_feedback` and `api::admin::tests::report_detail_rejects_unknown_report`) — separate semantic-drift problem tracked separately.
- Refactoring `test_state()` to not eagerly seed `relay.example` — a decoupling worth doing later so the upload-community-scope tests can run without PG at all. This `#[ignore]` change intentionally makes the *minimum* delta.

## Verification (after patch on a machine without PG running)

```
cargo test -p buzz-relay --lib --no-fail-fast
# → 826 passed; 2 failed; 42 ignored (the 2 admin fails are pre-existing)
# vs today's
# → 824 passed; 9 failed; 0 ignored
cargo test -p buzz-relay api::media -- --include-ignored
# → 7 media tests run and fail with PoolTimedOut, as expected when PG is down
```

Open crates/buzz-relay/src/api/media.rs and locate the seven named tests in its tests module; compare their annotations with crates/buzz-search/tests/fts_integration.rs. Apply the established ignored-test pattern, then run cargo test -p buzz-relay --lib --no-fail-fast without Postgres and confirm the seven media tests are ignored while the two noted admin failures remain.

Written by the indexing model from the issue text.