Record extension options and decisions in docs/ROADMAP.md

Nothing implemented. Captures the setup decisions (bearer token,
stateless, inline extraction) and, for any future persistence, that it
goes in the Pi's existing PostgreSQL under its own database and user.

Worth keeping because the build surfaced facts that are expensive to
rediscover: search covers file names but never file contents, so the 93
PDFs here are opaque to it; one search costs ~270 upstream requests;
extracted file text is immutable per fileRecord and so cacheable
forever; German needs the german FTS dictionary plus pg_trgm, since
compounds defeat stemming; and "what's new since X" is impossible today
because the API has no changed-since filter anywhere.

Also records what is ruled out and why — collaborative text editor
contents are not retrievable, OCR is unnecessary since images go to
Claude directly, and write tools would forfeit the read-only property
that makes the public endpoint acceptable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -83,6 +83,10 @@ per file element — because a model asking "what's on this board" wants the
 answer, not a traversal plan. Output is Markdown with ids preserved for
 follow-up calls, not raw JSON.
 
+Possible extensions — full-text search over file contents, a cache with a
+bypass, "what's new since…" — are sketched with their trade-offs in
+[docs/ROADMAP.md](docs/ROADMAP.md). None are built.
+
 ## Layout
 
 ```