ingest: Initial implementation

Bot that monitors a Matrix room for PDF and JPEG files and uploads
them to Paperless-ngx. Supports E2E encrypted attachments via inline
AES keys, historical catchup on startup, exponential backoff retries
with a permanent give-up after max attempts, file format validation
via magic bytes, Uptime Kuma heartbeat monitoring, and email alerts
on errors via SMTP SSL.

.env.example

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+MATRIX_HOMESERVER=https://jeena.net
+MATRIX_USER=@bot:jeena.net
+MATRIX_ACCESS_TOKEN=syt_...
+MATRIX_DEVICE_ID=ABCDEFGH
+MATRIX_ROOM_ID=!roomid:jeena.net
+
+PAPERLESS_URL=https://paperless.jeena.net
+PAPERLESS_TOKEN=your_paperless_api_token
+PAPERLESS_INBOX_TAG_ID=1
+
+# Optional: path to the SQLite state database (default: state.db next to the script)
+DB_PATH=state.db
+
+# Optional: Uptime Kuma push monitor URL for heartbeat monitoring
+# Create a Push monitor in Uptime Kuma and paste the URL here.
+# UPTIME_KUMA_PUSH_URL=https://uptime.example.com/api/push/xxxx?status=up&msg=OK&ping=
+
+# Optional: email alerts on errors
+# SMTP_HOST=mail.jeena.net
+# SMTP_PORT=465
+# SMTP_USER=user@jeena.net
+# SMTP_PASSWORD=yourpassword
+# ALERT_FROM=paperless-ingest@jeena.net
+# ALERT_TO=you@jeena.net