fix(ingestion): address code review issues in api and watcher packages

- Strip internal error detail from 500 responses (leak prevention)
- Add path containment assertion in /write handler
- Use Go 1.22 method-prefixed mux routes for automatic 405 responses
- Clarify watch_interval log when watcher not yet wired
- Consolidate validation tests into table-driven TestIngest_Validation
- Watcher: return nil after successful quarantine to avoid double-logging
- Watcher: append timestamp suffix to processed dest if file already exists

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ingestion/cmd/server/main.go

@@ -2,6 +2,7 @@
 package main
 
 import (
+	"fmt"
 	"log/slog"
 	"net/http"
 	"os"
@@ -55,19 +56,23 @@ func main() {
 	_ = watchInterval
 
 	mux := http.NewServeMux()
-	mux.HandleFunc("/query", h.Query)
-	mux.HandleFunc("/write", h.Write)
-	mux.HandleFunc("/ingest", h.Ingest)
-	mux.HandleFunc("/ingest-path", h.IngestPath)
+	mux.HandleFunc("POST /query", h.Query)
+	mux.HandleFunc("POST /write", h.Write)
+	mux.HandleFunc("POST /ingest", h.Ingest)
+	mux.HandleFunc("POST /ingest-path", h.IngestPath)
 
 	addr := ":" + port
+	watchIntervalLog := "disabled"
+	if watchInterval > 0 {
+		watchIntervalLog = fmt.Sprintf("%ds (pending task11 wiring)", watchInterval)
+	}
 	logger.Info("ingestion server starting",
 		"addr", addr,
 		"brain_dir", brainDir,
 		"llm_url", llmURL,
 		"llm_model", llmModel,
 		"chunk_size", chunkSize,
-		"watch_interval_s", watchInterval,
+		"watch_interval", watchIntervalLog,
 	)
 	if err := http.ListenAndServe(addr, mux); err != nil {
 		logger.Error("server stopped", "err", err)