1. sms.py: Replace per-send aiohttp.ClientSession with a persistent
session created in connect() and closed in disconnect(). Each
outbound SMS no longer pays the TCP+TLS handshake cost. Falls back
to a temporary session if the persistent one isn't available.
2. matrix.py: Use proper MIME types (image/png, audio/ogg, video/mp4)
instead of bare category words (image, audio, video). The gateway's
media processing checks startswith('image/') and startswith('audio/')
so bare words caused Matrix images to skip vision enrichment and
Matrix audio to skip transcription. Now extracts the actual MIME
type from the nio event's content info when available.
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