feat(reader): open DjVu documents in the web reader

Scanned books are commonly distributed as DjVu, which the scanner only stored as a supplement.
Decode pages on the server with DjVuLibre so DjVu imports with a cover and page count, opens in
the image reader with its embedded OCR text, and can be the preferred format of an ordinary email
recipient. Migration 0113 widens the recipient format check to allow it.

Decoding runs on validated private snapshots behind a bounded, reader-first queue, and
revalidation and cached pages are answered without a decoder. Released iOS builds cannot open
DjVu, so series continuation offers it only to callers sending includeDjvu=true, and the
dashboard and notebook read file never name it. The first full scan after upgrading promotes
stored .djvu supplements to content.

server/.env.example

@@ -48,6 +48,12 @@ COVER_SLOTS_BACKFILL_MODE=background
 # FILE_WRITE_DEBOUNCE_MS=3000
 # FILE_WRITE_MAX_CONCURRENT_WRITES=2
 
+# DjVuLibre reader limits. ddjvu and djvused must be on PATH.
+# DJVU_MAX_CONCURRENT=2
+# DJVU_TIMEOUT_MS=30000
+# DJVU_MAX_SOURCE_BYTES=268435456
+# DJVU_CACHE_BYTES=67108864
+
 # Podcast security and resource limits
 # Required in production. Development falls back to JWT_SECRET when unset.
 # Existing production installs must initially use their current JWT_SECRET so encrypted feed URLs remain readable.