docs(release): 0.13.1 release notes

[lesnik512]

Notes for the 0.13.1 patch, which ships the fastapi<0.140 cap merged in #153.

The cap is currently unreleased, so it protects nobody — published 0.13.0 still carries fastapi>=0.95, and a fresh install resolves the broken pairing:

$ uv pip install --dry-run 'faststream-outbox[fastapi]'
 + fastapi==0.140.1
 + faststream==0.7.2
 + faststream-outbox==0.13.0

Release notes are mandatory for a stable tag — the workflow checks for planning/releases/<tag>.md before just publish, so a missing file aborts the release before anything reaches PyPI.

Notes-only change; no package code touched. just lint-ci green locally.

🤖 Generated with Claude Code

[github-actions]

Benchmark gate

✅ gate passed

scenario	msg/s	delete/msg	WALrec/msg	WALB/msg	fpi	upd	del	dead_tup
consumer/w1/b10	955	1.000	6.79	918	242	5000	5000	10000
consumer/w1/b100	964	1.000	6.84	919	243	5000	5000	10000
consumer/w2/b10	1145	1.000	6.81	966	242	5000	5000	10000
consumer/w2/b100	1372	1.000	6.73	957	243	5000	5000	10000
consumer/w4/b10	1048	1.000	6.77	960	242	5000	5000	10000
consumer/w4/b100	1575	1.000	6.86	1033	244	5000	5000	10000
consumer/w1/b100/tfbs100	5479	0.010	6.07	1116	243	5000	5000	10000
producer/w1/b100	2137	0.000	3.04	584	0	0	0	0

Gated (fails the build): delete_calls + tuple counters (upd/del/ins) + the producer's insert_calls/select_calls, exact; wal_records within a 10% band. msg/s, WAL bytes and total calls are informational (timing/FPI noise).