Add resizable room panel, searchable user picker, and direct room membership

Room info panel is now user-resizable (fixing a layout clip at narrow
widths), and every user-selection spot (room membership, admin ownership
transfer) uses a new searchable UserPicker instead of raw text input or
prompt(). Member rows fold role + actions into a single inline dropdown
instead of a row of buttons, so the member list stays usable as rooms grow.

Room invites (the accept/decline flow) are replaced by adding a user to a
room directly -- an admin/owner picks someone and they're a member
immediately, with a "you've been added" notification email instead of an
invite email. Drops the now-unused room_invites table.

backend/app/schemas/invite.py

@@ -1,31 +0,0 @@
-import uuid
-from datetime import datetime
-
-from pydantic import BaseModel, ConfigDict, Field
-
-from app.models import InviteStatus
-
-
-class InviteCreate(BaseModel):
-    target_username: str = Field(min_length=1)
-
-
-class InviteRead(BaseModel):
-    model_config = ConfigDict(from_attributes=True)
-
-    id: uuid.UUID
-    room_id: uuid.UUID
-    invited_by: uuid.UUID
-    target_user_id: uuid.UUID | None
-    target_username: str | None = None
-    status: InviteStatus
-    expires_at: datetime
-    created_at: datetime
-
-
-class MyInviteRead(InviteRead):
-    """InviteRead plus context the recipient can't otherwise resolve client-side --
-    GET /api/invites/mine is for rooms the user isn't a member of yet."""
-
-    room_name: str
-    invited_by_username: str

backend/app/schemas/room.py

@@ -45,6 +45,10 @@ class RoomMemberRead(BaseModel):
     joined_at: datetime
 
 
+class RoomMemberAdd(BaseModel):
+    user_id: uuid.UUID
+
+
 class RoomMemberRoleUpdate(BaseModel):
     role: RoomRole
 

backend/app/schemas/user.py

@@ -25,3 +25,17 @@ class UserRead(BaseModel):
 
 class ProfileUpdate(BaseModel):
     display_name: str | None = Field(default=None, max_length=50)
+
+
+class UserDirectoryRead(BaseModel):
+    """Lightweight entry for user-picker UIs (room invites, admin ownership
+    transfer) -- same visibility level as an avatar: any authenticated user
+    can see this much about anyone (excludes bots, which aren't invited
+    through these flows)."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    username: str
+    display_name: str | None
+    avatar_filename: str | None