feat(users): UserManager with per-user SQLCipher, and extract skald-core crate

Two changes developed together in one session; they share the same module
structure (db/mod.rs, the core lib root) and only compile together, so they
land as one commit.

## UserManager + per-user encryption (§9/§11)

New `users::UserManager`: owns the system.db pool plus a map
`userid -> SqlitePool` of unlocked databases. The pool *is* the unlock token —
its connect options carry the DEK as SQLCipher's raw key, so an open pool means
the key is in RAM until restart and dropping it re-locks (§9). Knows nothing
about cookies.

New `crypto` module: envelope encryption. A random 256-bit DEK encrypts
`{userid}.db`; `users.database_password` holds it sealed with AES-256-GCM under
`Argon2id(password, salt)`. The AEAD tag is the password verifier — one
derivation both authenticates and yields the key, so encrypted users store no
second hash. Cleartext users store the Argon2id output directly, compared in
constant time. Argon2 runs in spawn_blocking behind a 2-permit semaphore
(256 MiB per derivation).

- SQLCipher via `libsqlite3-sys` `bundled-sqlcipher-vendored-openssl`, pinned
  <0.38 so it unifies with the one sqlx-sqlite links (a newer copy would apply
  the feature to a SQLite sqlx never uses). OpenSSL is vendored and static, so
  the binary stays self-contained.
- Schema split into `create_registry_tables` (instance-wide, no user key) and
  `create_owner_tables` (one owner's content, identical in every file). No FK in
  the owner bucket may reach the registry — enforced by a standalone test.
  Dropped `chat_history.model_db_id` (write-only, and the only registry-crossing
  key); moved `projects`/`project_tickets` into the owner bucket.
- Provisioning invariant: the file is written before the row, deleted after it,
  so a crash leaves an orphan file, never a user without a database. `open_db`
  never creates: a missing file is an error, not a silent empty database.

Not consumed yet: no login, call sites still use the shared system.db pool.

## Extract crates/skald-core

The headless core moves out of `src/` into its own crate; `skald` (server) and
the coming `skald-setup` are shells around it. Two dependencies on the shell
were inverted rather than dragged along, so the core names neither Tauri nor any
concrete plugin:

- `Plugin::tools(self: Arc<Self>)` — plugins contribute tools through this hook
  (sibling of `http_router`), so the core no longer downcasts to
  `MobileConnectorPlugin`.
- `tools::restart::set_restart_handler` — the desktop shell installs its
  teardown-and-respawn; the core defaults to the supervisor exit code. The core
  loses its `desktop` feature.
- `boot`'s stdout formatter moves to the binary (`src/boot_format.rs`); the core
  only emits tracing events.

All 79 core tests pass; the binary boots and serves in a clean directory, and
the mobile-connector tools still register through the new hook.

src/frontend/api/ws.rs

@@ -12,9 +12,9 @@ use serde_json::Value;
 use tokio::sync::broadcast;
 use tracing::{debug, info, warn};
 
-use crate::core::chat_hub::{ModelCommandOutcome, SendMessageOptions};
-use crate::core::events::{ClientMessage, ServerEvent};
-use crate::core::skald::Skald;
+use skald_core::chat_hub::{ModelCommandOutcome, SendMessageOptions};
+use skald_core::events::{ClientMessage, ServerEvent};
+use skald_core::skald::Skald;
 use core_api::command::CommandApi;
 
 #[derive(Deserialize)]
@@ -359,7 +359,7 @@ async fn handle_socket(mut socket: WebSocket, skald: Arc<Skald>, source: String)
                     // viewer. Injected here (not in the registry) so it exists only
                     // for ws.rs clients (web + mobile), never for the Telegram plugin.
                     interface_tools: vec![
-                        crate::core::tools::show_file::make_tool(
+                        skald_core::tools::show_file::make_tool(
                             Arc::clone(skald.chat_hub()),
                             source.clone(),
                         ),
@@ -418,7 +418,7 @@ fn is_resume_msg(text: &str) -> bool {
 /// Returns true if the message was an approval/rejection (caller should `continue`).
 async fn handle_approval_msg(
     text:      &str,
-    chat_hub:  &Arc<crate::core::chat_hub::ChatHub>,
+    chat_hub:  &Arc<skald_core::chat_hub::ChatHub>,
 ) -> bool {
     let Ok(v) = serde_json::from_str::<Value>(text) else { return false };
     let Some(request_id) = v["request_id"].as_i64() else { return false };
@@ -445,7 +445,7 @@ async fn handle_approval_msg(
 /// Returns true if the message was a question answer (caller should `continue`).
 async fn handle_question_answer_msg(
     text:    &str,
-    handler: &Arc<crate::core::session::handler::ChatSessionHandler>,
+    handler: &Arc<skald_core::session::handler::ChatSessionHandler>,
 ) -> bool {
     let Ok(v) = serde_json::from_str::<Value>(text) else { return false };
     if v["type"].as_str() != Some("answer_question") { return false }
@@ -462,7 +462,7 @@ async fn handle_question_answer_msg(
 async fn handle_select_client_msg(
     text:     &str,
     source:   &str,
-    chat_hub: &Arc<crate::core::chat_hub::ChatHub>,
+    chat_hub: &Arc<skald_core::chat_hub::ChatHub>,
 ) -> bool {
     let Ok(v) = serde_json::from_str::<Value>(text) else { return false };
     if v["type"].as_str() != Some("select_client") { return false }
@@ -482,7 +482,7 @@ fn handle_data_msg(text: &str, skald: &Arc<Skald>) -> bool {
     let Ok(v) = serde_json::from_str::<Value>(text) else { return false };
     if v["type"].as_str() != Some("data") { return false }
 
-    let Ok(msg) = serde_json::from_value::<crate::core::events::InboundDataMessage>(v) else {
+    let Ok(msg) = serde_json::from_value::<skald_core::events::InboundDataMessage>(v) else {
         return true;
     };
 
@@ -495,7 +495,7 @@ fn handle_data_msg(text: &str, skald: &Arc<Skald>) -> bool {
             if let (Some(lat), Some(lng)) = (lat, lng) {
                 skald.location_manager().update(
                     "remote",
-                    crate::core::location::GpsCoord { latitude: lat, longitude: lng },
+                    skald_core::location::GpsCoord { latitude: lat, longitude: lng },
                     acc,
                     live,
                 );