Add Discord outbound posting (#14)

Stacks on #13. Adds discord_post_links and a subscriber that posts roots into a public thread, replies into that thread, and edits the linked Discord message. Unset Discord env leaves the site unchanged.

Reviewed-on: #14
Co-authored-by: codegirl-007 <s.raide@gmail.com>

go.mod

@@ -7,10 +7,12 @@ require (
 	github.com/aws/aws-sdk-go-v2 v1.43.7
 	github.com/aws/aws-sdk-go-v2/credentials v1.19.37
 	github.com/aws/aws-sdk-go-v2/service/s3 v1.107.3
+	github.com/bwmarrin/discordgo v0.29.0
 	github.com/go-chi/chi/v5 v5.3.1
 	github.com/google/uuid v1.6.0
 	github.com/jackc/pgx/v5 v5.10.0
 	github.com/joho/godotenv v1.5.1
+	github.com/resend/resend-go/v3 v3.16.0
 	golang.org/x/crypto v0.55.0
 	golang.org/x/image v0.45.0
 )
@@ -25,10 +27,11 @@ require (
 	github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.13.38 // indirect
 	github.com/aws/aws-sdk-go-v2/service/internal/s3shared v1.19.39 // indirect
 	github.com/aws/smithy-go v1.27.8 // indirect
+	github.com/gorilla/websocket v1.4.2 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
 	github.com/jackc/puddle/v2 v2.2.2 // indirect
-	github.com/resend/resend-go/v3 v3.16.0 // indirect
 	golang.org/x/sync v0.22.0 // indirect
+	golang.org/x/sys v0.47.0 // indirect
 	golang.org/x/text v0.41.0 // indirect
 )