uploads: centralise via ChatHubApi::save_upload, refactor handlers

Extract shared upload seam in skald-core, move Telegram and web
handlers to use it. Simplify media attachment routing. Clean up
unused deps and dead code.

crates/core-api/src/chat_hub.rs

@@ -5,7 +5,7 @@ use tokio::sync::broadcast;
 
 use crate::events::GlobalEvent;
 use crate::interface_tool::InterfaceTool;
-use crate::message_meta::MessageMetadata;
+use crate::message_meta::{Attachment, MessageMetadata};
 
 // ── SendMessageOptions ────────────────────────────────────────────────────────
 
@@ -59,6 +59,20 @@ pub trait ChatHubApi: Send + Sync {
         opts: SendMessageOptions,
     ) -> anyhow::Result<()>;
 
+    /// Persist an uploaded file for `source_id` into the owner's
+    /// `~/uploads/{session}/` and return its [`Attachment`] (home-relative agent
+    /// path). Channel adapters (e.g. the Telegram plugin) call this instead of
+    /// writing files themselves, so the core owns *where* uploads land and every
+    /// surface produces a path the agent can actually reach. The recognized
+    /// magic-byte MIME wins over the caller-claimed `client_mime`.
+    async fn save_upload(
+        &self,
+        source_id: &str,
+        file_name: &str,
+        client_mime: Option<String>,
+        bytes: &[u8],
+    ) -> anyhow::Result<Attachment>;
+
     /// Create a new session for the source, discarding the previous one.
     async fn clear(&self, source_id: &str) -> anyhow::Result<i64>;
 

crates/core-api/src/message_meta.rs

@@ -12,9 +12,10 @@
 
 use serde::{Deserialize, Serialize};
 
-/// One file attached by the user to a message. `path` is relative to the project
-/// root (e.g. `data/uploads/123/file.pdf`) so it is both servable under `/data/…`
-/// and resolvable by the filesystem tools.
+/// One file attached by the user to a message. `path` is a home-relative agent
+/// path (e.g. `uploads/123/file.pdf`) — the caller's container home is its root,
+/// so the fs-tools, `execute_cmd`, the file viewer (`/api/file`) and the media
+/// inliner all resolve it to the same physical file.
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct Attachment {
     pub path:     String,

crates/core-api/src/user_fs.rs

@@ -22,6 +22,12 @@
 use std::path::{Component, Path, PathBuf};
 use std::sync::{Arc, RwLock};
 
+/// The subdirectory of a user's home where chat uploads are saved
+/// (`{home}/uploads/{session_id}/…`, reachable by the agent as `uploads/…`).
+/// Shared by the upload handler (write path) and the media inliner (containment
+/// root) so the two anchors can never drift.
+pub const UPLOADS_SUBDIR: &str = "uploads";
+
 /// One shared folder mounted into a user's container.
 #[derive(Debug, Clone)]
 pub struct SharedMount {