Add #roomname references in chat messages (#47)

Mirrors the existing @-mention system's shape: a regex finds #roomname
tokens, extract_referenced_room_ids validates them against rooms the
*sender* actually belongs to (mirrors mentions' "must be a real member"
rule -- referencing a private room the sender isn't in would otherwise
leak its existence), and a MessageRoomReference join row is stored per
match in create_message. No notification/unread layer, unlike mentions --
referencing a room has no "you were referenced" semantics.

Rendering is the same markdown-link rewrite trick MessageContent.tsx
already uses for mentions (#username -> [#username](mention:username)),
but resolved against the *viewer's* own room list (threaded down from
ChatShellPage's room state through ChatPane/MessageList) rather than the
stored server-side reference -- a reference to a room the current viewer
isn't in quietly renders as plain text instead of a link, same as an
@mention of someone outside the room does. The href scheme renders a
real react-router Link instead of mentions' inert span, since a room
reference is meant to be navigable.

mention_service.strip_code_spans (was _strip_code_spans) is now shared
between both extraction paths rather than private to one module.

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

backend/app/models/__init__.py

@@ -11,6 +11,7 @@ from app.models.message_file import MessageFile
 from app.models.message_image import MessageImage
 from app.models.message_mention import MessageMention
 from app.models.message_reaction import MessageReaction
+from app.models.message_room_reference import MessageRoomReference
 from app.models.password_reset import PasswordReset
 from app.models.push_subscription import PushSubscription
 from app.models.room import Room
@@ -31,6 +32,7 @@ __all__ = [
     "MessageImage",
     "MessageMention",
     "MessageReaction",
+    "MessageRoomReference",
     "InviteStatus",
     "PasswordReset",
     "SiteInvite",

backend/app/models/message_room_reference.py

@@ -0,0 +1,15 @@
+import uuid
+
+from sqlalchemy import ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.models.base import Base
+
+
+class MessageRoomReference(Base):
+    __tablename__ = "message_room_references"
+
+    message_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("messages.id"), primary_key=True)
+    room_id: Mapped[uuid.UUID] = mapped_column(
+        ForeignKey("rooms.id"), primary_key=True, index=True
+    )