refactor(store,tools): migrate IDs from UUID to bigserial int64

All internal entity lookups now use bigserial primary keys (int64) while
GUIDs are retained for external/public identification. Updated store
functions (TouchProject, UpdateThoughtMetadata, AddThoughtAttachment) to
query by id instead of guid, added GetThoughtByID, changed semanticSearch
and all tool helpers to use *int64 project IDs, and updated retry/backfill
workers to use int64 thought IDs throughout.

internal/tools/helpers.go

@@ -3,6 +3,7 @@ package tools
 import (
 	"context"
 	"fmt"
+	"strconv"
 	"strings"
 
 	"github.com/google/uuid"
@@ -34,6 +35,14 @@ func parseUUID(id string) (uuid.UUID, error) {
 	return parsed, nil
 }
 
+func parseID(id string) (int64, error) {
+	n, err := strconv.ParseInt(strings.TrimSpace(id), 10, 64)
+	if err != nil {
+		return 0, fmt.Errorf("invalid numeric id %q", id)
+	}
+	return n, nil
+}
+
 func sessionID(req *mcp.CallToolRequest) (string, error) {
 	if req == nil || req.Session == nil || req.Session.ID() == "" {
 		return "", newMCPError(