docs: document SIP_ENGINE and correct config tables against the models

Started as the SIP_ENGINE row flagged in the last commit. Cross-checking the
tables against config.py mechanically (rather than by eye) turned up more,
including two entries that were actively wrong.

Corrections:
- GATEWAY_RTP_PORT_MIN/MAX and GATEWAY_HOST are documented in
  configuration.md but do not exist — no code reads them and they are absent
  from .env.example. Setting them today does nothing. Replaced with the real
  GATEWAY_SIP_ fields (host/port/domain).
- GATEWAY_SIP_PORT was documented as 5080 in two places; the code and
  .env.example both say 5060.
- DATABASE_URL was documented with a SQLite default. There is none, and
  startup exits if it is unset.

Additions — every env var the models accept is now documented somewhere
(verified bidirectionally: nothing in the models undocumented, nothing
documented that the models reject):
- Server section: HOST, PORT, DEBUG, LOG_LEVEL, LOG_FORMAT
- Safety section: MAX_CONCURRENT_CALLS, USE_MOCK_SIP, SIP_ENGINE
- Receptionist section (configuration.md had none, though seven vars exist)

Structural staleness, from the PR #8 media-plane work:
- core/pjsua_engine.py was absent from the component list and file tree; so
  were dial_plan.py (the emergency guard) and sip_engine.py.
- architecture.md's banner still read "media plane in transition". The engine
  landed; it is now two selectable engines with the audio consequence stated.
- Tech Stack described "single-process async architecture" — the
  simplification CLAUDE.md explicitly calls out. Now points at the threading
  model, since there are three execution contexts.
- The Asterisk lab shipped in PR #8 with its own README but nothing linked to
  it. Linked from the test section and both doc indexes.
- CLAUDE.md's "no structured JSON logging" gap is closed; test count was 146
  across 16 files, now 189 across 19. The other listed gaps (no /metrics, no
  rate limiting, no health-probe log filter) were re-verified and still hold.

Deliberately not hardcoding a test count in the README — that is the same
staleness this commit is clearing up. All internal links and anchors verified
to resolve; 189 tests pass; lint unchanged at its 216 baseline.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/README.md

@@ -8,6 +8,9 @@ Comprehensive documentation for the Hold Slayer AI telephony gateway.
 |----------|-------------|
 | [Architecture](architecture.md) | System architecture, component diagram, data flow |
 | [Core Engine](core-engine.md) | SIP engine, media pipeline, call manager, event bus |
+| [Dial Plan](dial-plan.md) | Number normalisation and the emergency-number guard |
+| [PJSUA2 Build](pjsua2-build.md) | Building the `pjsua2` bindings (not pip-installable) |
+| [Asterisk Lab](../tests/lab/README.md) | The fake PSTN used for media validation |
 | [Hold Slayer Service](hold-slayer-service.md) | IVR navigation, hold detection, human detection, transfer |
 | [Audio Classifier](audio-classifier.md) | Waveform analysis, feature extraction, classification logic |
 | [Services](services.md) | LLM client, transcription, recording, analytics, notifications |