fix(schedules): use typed error for pause/resume 404 discrimination

Replace brittle msg.contains("not found") string matching in
post_schedule_pause / post_schedule_resume with a typed
ScheduleNotFoundOrCancelled error that handlers downcast on directly.

pause() and resume() now return Err(ScheduleNotFoundOrCancelled(id).into())
instead of bail!("schedule {id} not found or is cancelled"); handlers call
e.downcast_ref::<ScheduleNotFoundOrCancelled>().is_some() for the 404 branch,
making the discrimination stable even if the error message wording changes.

hive-c0re/src/scheduled_prompts.rs

@@ -19,6 +19,20 @@ use anyhow::{Context, Result, bail};
 use rusqlite::{Connection, OptionalExtension, params};
 use serde::{Deserialize, Serialize};
 
+/// Typed error returned by [`ScheduledPrompts::pause`] and
+/// [`ScheduledPrompts::resume`] when the target row does not exist or
+/// is already cancelled. Handlers downcast on this type to emit 404
+/// rather than 500, avoiding brittle string-matching on the message.
+#[derive(Debug)]
+pub struct ScheduleNotFoundOrCancelled(pub i64);
+
+impl std::fmt::Display for ScheduleNotFoundOrCancelled {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "schedule {} not found or is cancelled", self.0)
+    }
+}
+impl std::error::Error for ScheduleNotFoundOrCancelled {}
+
 const SCHEMA: &str = r"
 CREATE TABLE IF NOT EXISTS scheduled_prompts (
     id                INTEGER PRIMARY KEY AUTOINCREMENT,
@@ -577,7 +591,8 @@ impl ScheduledPrompts {
     /// `next_fire_at_unix` is preserved so the first resume fires at
     /// the next intended cadence instant (no catch-up needed — a
     /// paused schedule simply slips its upcoming fire). Returns
-    /// `Err` when the schedule is cancelled or does not exist.
+    /// `Err(ScheduleNotFoundOrCancelled)` when the schedule is
+    /// cancelled or does not exist (handlers downcast to emit 404).
     pub fn pause(&self, id: i64) -> Result<()> {
         let conn = self.conn.lock().unwrap();
         let now = now_unix();
@@ -588,13 +603,14 @@ impl ScheduledPrompts {
             params![now, id],
         )?;
         if n == 0 {
-            bail!("schedule {id} not found or is cancelled");
+            return Err(ScheduleNotFoundOrCancelled(id).into());
         }
         Ok(())
     }
 
     /// Resume a paused schedule. Idempotent; no-op on an active row.
-    /// Returns `Err` when the schedule is cancelled or does not exist.
+    /// Returns `Err(ScheduleNotFoundOrCancelled)` when the schedule is
+    /// cancelled or does not exist (handlers downcast to emit 404).
     pub fn resume(&self, id: i64) -> Result<()> {
         let conn = self.conn.lock().unwrap();
         let n = conn.execute(
@@ -604,7 +620,7 @@ impl ScheduledPrompts {
             params![id],
         )?;
         if n == 0 {
-            bail!("schedule {id} not found or is cancelled");
+            return Err(ScheduleNotFoundOrCancelled(id).into());
         }
         Ok(())
     }