feat(webrtc): end-to-end encrypted voice & video calls with adaptive codecs

Add 1:1 voice and video calling over the existing SAS-verified peer
connection. Audio and video tracks ride the same RTCPeerConnection as the
chat, bundled onto one DTLS-SRTP transport, so media inherits the session's
end-to-end encryption. SDP offer/answer is renegotiated in-band over the
verified data channel — no signalling server, so the media's DTLS
fingerprints are authenticated end-to-end. Calls are gated on a connected,
SAS-verified session.

Codecs & adaptation:
- Opus tuned for lossy links (in-band FEC, DTX, RED redundancy); audio is
  bandwidth-prioritised and never throttled.
- VP9/AV1 single-encoding SVC with H.264/VP8 fallback; video degrades by
  spatial/temporal layer.
- Runtime NetworkAdaptationController trims video bitrate on loss/RTT and
  recovers as the link clears — no renegotiation. Live connection-quality
  indicator (Excellent/Good/Fair/Weak) in the call UI.

In-call controls: mute, camera on/off (voice→video upgrade in-band),
camera flip, minimize-to-widget, hang up, and accept/decline for incoming
calls. Production logging disabled (DEBUG_MODE=false); temporary call
diagnostic logger removed. Codec rationale in docs/webrtc-config.md.

.htaccess

@@ -145,8 +145,8 @@
     Header set X-Frame-Options "DENY"
     # Force HTTPS (2 years + preload) to close the first-visit SSL-strip window.
     Header set Strict-Transport-Security "max-age=63072000; includeSubDomains; preload"
-    # Restrict powerful features; camera kept for in-page QR scanning.
-    Header set Permissions-Policy "camera=(self), microphone=(), geolocation=(), payment=(), usb=(), magnetometer=(), gyroscope=(), accelerometer=()"
+    # Restrict powerful features; camera + microphone kept for QR scanning and calls.
+    Header set Permissions-Policy "camera=(self), microphone=(self), geolocation=(), payment=(), usb=(), magnetometer=(), gyroscope=(), accelerometer=()"
 </IfModule>
 
 # Content Security Policy (frame-ancestors and report-uri only work in HTTP headers, not meta tags)