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feat(bb_event_routes): tighten default queue_depth and max_clients for tight-heap boards (#258)

Audit (B1-204) confirmed: with current defaults queue_depth=32 and max_clients=4,
each SSE client consumes ~9 KB heap (8 KB payload buf + 768 B queue entries + locks).
On tight-heap boards without PSRAM (~30 KB budget for idle TLS handshake), four
clients fragment heap such that mbedtls_ssl_setup (~30 KB contiguous) fails.
Bench evidence on bitaxe-650: largest_free_block dropped to 3,444 B after 2 SSE
clients + stratum activity.

New defaults (queue_depth=8, max_clients=2) reduce per-client consumption to ~2.7 KB
(8×256 B payload buf + 8×24 B queue entry overhead + ~88 B mutex), leaving contiguous
heap available for TLS handshakes and other transients. Consumers with larger heaps
can override via sdkconfig. Updated Kconfig help text and CLAUDE.md Event bus section
to document the heap budget and per-client overhead.

Fixes B1-204 for ESP32-S3 without PSRAM and similar tight-heap boards.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

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