Fix correctness bugs found by code review

- matrix: drop cryptohelper LoginAs so device.json creds aren't
  invalidated on every restart by Init's re-login
- matrix: thread ctx through PostThreadedReply; bot dispatcher derives
  per-command ctx from app lifecycle so SIGTERM cancels in-flight work
- matrix: isTokenValid verifies /whoami user_id matches configured one
- matrix: loadDevice distinguishes missing file from corrupt parse;
  refuse to silently overwrite a corrupt device.json
- matrix: Start checks Syncer type assertion and returns error
- arr: parseLookup skips empty-title items and extracts existing id
- bot: skip Add when Result.Exists; reply "already in library"

internal/arr/arr.go

@@ -19,11 +19,15 @@ import (
 
 // Result is a single candidate returned by an *arr lookup. Title and Year
 // are extracted for display; raw is the untouched JSON object used to build
-// the Add payload.
+// the Add payload. Exists is true when the *arr server already has this item
+// in its library (lookup responses set "id" to a non-zero value in that case),
+// so the bot can skip the Add and report 'already in library' instead of
+// blindly re-POSTing.
 type Result struct {
-	Title string
-	Year  int
-	raw   json.RawMessage
+	Title  string
+	Year   int
+	Exists bool
+	raw    json.RawMessage
 }
 
 // Raw exposes the underlying lookup JSON; useful for tests and debugging.
@@ -105,10 +109,29 @@ func parseLookup(body []byte, titleField string) ([]Result, error) {
 			continue
 		}
 		var title string
-		_ = json.Unmarshal(meta[titleField], &title)
+		if raw, ok := meta[titleField]; ok && len(raw) > 0 {
+			_ = json.Unmarshal(raw, &title)
+		}
+		if title == "" {
+			// Skip items we can't display. The Add path would still POST the
+			// raw JSON, but the user reply would read "Added to Radarr:" with
+			// no title — worse than dropping the candidate.
+			continue
+		}
 		var year int
-		_ = json.Unmarshal(meta["year"], &year)
-		out = append(out, Result{Title: title, Year: year, raw: item})
+		if raw, ok := meta["year"]; ok && len(raw) > 0 {
+			_ = json.Unmarshal(raw, &year)
+		}
+		var existingID int
+		if raw, ok := meta["id"]; ok && len(raw) > 0 {
+			_ = json.Unmarshal(raw, &existingID)
+		}
+		out = append(out, Result{
+			Title:  title,
+			Year:   year,
+			Exists: existingID > 0,
+			raw:    item,
+		})
 	}
 	return out, nil
 }