feat(db): add guild_settings table for per-guild configuration

Store guild-specific settings (roles, channels, moderation options) in
database instead of config file, enabling per-guild configuration and
runtime updates without redeployment.

shared/db/migrations/0004_bored_kat_farrell.sql

@@ -0,0 +1,17 @@
+CREATE TABLE "guild_settings" (
+	"guild_id" bigint PRIMARY KEY NOT NULL,
+	"student_role_id" bigint,
+	"visitor_role_id" bigint,
+	"color_role_ids" jsonb DEFAULT '[]'::jsonb,
+	"welcome_channel_id" bigint,
+	"welcome_message" text,
+	"feedback_channel_id" bigint,
+	"terminal_channel_id" bigint,
+	"terminal_message_id" bigint,
+	"moderation_log_channel_id" bigint,
+	"moderation_dm_on_warn" jsonb DEFAULT 'true'::jsonb,
+	"moderation_auto_timeout_threshold" jsonb,
+	"feature_overrides" jsonb DEFAULT '{}'::jsonb,
+	"created_at" timestamp with time zone DEFAULT now() NOT NULL,
+	"updated_at" timestamp with time zone DEFAULT now() NOT NULL
+);