fix: bound event and channel queues

[cofin]

What changed

• add configurable drop-oldest capacity to PostgreSQL listener buffers
• bound Litestar channel output independently with depth and drop diagnostics
• validate the shared event setting across adapters and document transport behavior

Why

PostgreSQL listener consumers and the Litestar Channels backend used unbounded in-memory queues, allowing slow consumers to grow memory without limit.

Closes #698
Closes #699

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.87234% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.32%. Comparing base (851db3b) to head (5a6a1ef).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sqlspec/extensions/events/_buffer.py	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #700      +/-   ##
==========================================
- Coverage   77.40%   76.32%   -1.09%     
==========================================
  Files         486      487       +1     
  Lines       68307    68387      +80     
  Branches     9410     9418       +8     
==========================================
- Hits        52876    52197     -679     
- Misses      12023    12815     +792     
+ Partials     3408     3375      -33     

Flag	Coverage Δ
integration	58.80% <79.78%> (-2.05%)	⬇️
py3.10	75.81% <97.87%> (+0.09%)	⬆️
py3.11	75.83% <97.87%> (+0.10%)	⬆️
py3.12	75.83% <97.87%> (+0.09%)	⬆️
py3.13	74.48% <97.87%> (-1.27%)	⬇️
py3.14	64.26% <90.32%> (-12.35%)	⬇️
unit	65.14% <90.42%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sqlspec/adapters/asyncpg/events/_hub.py	76.74% <100.00%> (+1.43%)	⬆️
sqlspec/adapters/psqlpy/events/_hub.py	78.60% <100.00%> (+0.97%)	⬆️
sqlspec/adapters/psycopg/events/_hub.py	80.58% <100.00%> (-0.09%)	⬇️
sqlspec/config.py	93.88% <100.00%> (+0.01%)	⬆️
sqlspec/extensions/events/_channel.py	77.49% <100.00%> (-2.64%)	⬇️
sqlspec/extensions/events/_store.py	95.74% <100.00%> (+0.09%)	⬆️
sqlspec/extensions/litestar/channels.py	83.72% <100.00%> (+6.60%)	⬆️
sqlspec/extensions/events/_buffer.py	91.30% <91.30%> (ø)

... and 41 files with indirect coverage changes

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