docs(project): document the content-token endpoint and CONTENT_TOKEN_TTL

Add POST /files/{file_id}/content-token to the spec, note that the content
GET's access_token parameter also accepts a content token, and document the
CONTENT_TOKEN_TTL knob (default 6h) and its leak/revocation trade-off.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -54,6 +54,14 @@ JWT_SECRET=change-me-to-a-random-32-byte-secret
 JWT_ACCESS_TTL=15m
 JWT_REFRESH_TTL=720h
 
+# How long a content token is valid. It's a single-file capability the client
+# puts in a media URL to open/stream an original by link (e.g. a long video in a
+# new tab), so playback survives the short access-token expiry and session
+# rotation. Longer = fewer interruptions but a wider window in which a leaked URL
+# can read that one file; it can't be revoked before expiry. Keep it roughly as
+# long as a viewing session lasts.
+CONTENT_TOKEN_TTL=6h
+
 # Reverse-proxy hops (comma-separated CIDRs/IPs) whose X-Forwarded-For is trusted,
 # so the auth rate limiter sees real client IPs instead of the proxy's. The default
 # covers loopback and the Docker bridge ranges a host nginx reaches the container