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/search.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"strings"
 
-	"github.com/google/uuid"
 	"github.com/modelcontextprotocol/go-sdk/mcp"
 
 	"git.warky.dev/wdevs/amcs/internal/ai"
@@ -50,10 +49,10 @@ func (t *SearchTool) Handle(ctx context.Context, req *mcp.CallToolRequest, in Se
 		return nil, SearchOutput{}, err
 	}
 
-	var projectID *uuid.UUID
+	var projectID *int64
 	if project != nil {
-		projectID = &project.ID
-		_ = t.store.TouchProject(ctx, project.ID)
+		projectID = &project.NumericID
+		_ = t.store.TouchProject(ctx, project.NumericID)
 	}
 
 	results, err := semanticSearch(ctx, t.store, t.embeddings, t.search, query, limit, threshold, projectID, nil)