perf(voice): make _TTS_PIPELINE_DEPTH env-configurable; default 2 (#6811)

[mrveiss]

Summary

_TTS_PIPELINE_DEPTH was hardcoded to 3 (#6752). Pocket TTS calls run via loop.run_in_executor(None, _run_synthesis, ...) and serialize on the GIL/model lock — depth>2 only adds queue contention without true parallelism.

This PR:

• Lowers default to 2 (one chunk synthesizes ahead while the client plays the current one — the actual jitter-absorption goal of bug(voice): TTS streaming stutters between sentence chunks — pipelining depth of 1 may be insufficient #6752)
• Adds AUTOBOT_TTS_PIPELINE_DEPTH env var override
• Clamps to 1..8 to prevent ops typos OOMing the worker
• Logs a warning + falls back to 2 on invalid values

Verification

$ python3 -c '... resolver tests ...'
Invalid AUTOBOT_TTS_PIPELINE_DEPTH='banana'; using 2
All resolver cases pass

Edge cases tested:

• default → 2
• '1' → 1 (min)
• '8' → 8 (max)
• '99' → 8 (upper clamp)
• '0' → 1 (lower clamp)
• '-5' → 1
• 'banana' → 2 + warning

python -m py_compile passes.

Test plan

• Deploy with default — voice latency improves vs current depth=3
• Set AUTOBOT_TTS_PIPELINE_DEPTH=1 (sequential) and =4 (over-saturated) in dev to compare
• Verify backend log line Invalid AUTOBOT_TTS_PIPELINE_DEPTH=... on bad value
• No regression in bug(voice): TTS streaming stutters between sentence chunks — pipelining depth of 1 may be insufficient #6752 jitter absorption (playback still gapless on multi-sentence responses)

Closes

Closes #6811

Related: #6810 (single-WS unification), #6752 (original jitter-absorption introducing depth=3)

[github-actions]

⚠️ SSOT Configuration Compliance: Violations Found

Metric	Count
Total Violations	1
SSOT Violations (high priority)	1
Other Violations	0

⚠️ 1 values have SSOT config equivalents!

These should be replaced with SSOT config imports:

Python:

from src.config.ssot_config import config
# Use: config.vm.main, config.port.backend, config.backend_url

TypeScript:

import config from '@/config/ssot-config'
// Use: config.vm.main, config.port.backend, config.backendUrl

📖 See SSOT_CONFIG_GUIDE.md for documentation.