Add MediaAsset model.

A MediaAsset represents an image or video file uploaded to Danbooru. It
stores the metadata associated with the image or video. This is to work
on decoupling files from posts so that images can be uploaded separately
from posts.

db/structure.sql

@@ -2461,6 +2461,41 @@ CREATE SEQUENCE public.ip_geolocations_id_seq
 ALTER SEQUENCE public.ip_geolocations_id_seq OWNED BY public.ip_geolocations.id;
 
 
+--
+-- Name: media_assets; Type: TABLE; Schema: public; Owner: -
+--
+
+CREATE TABLE public.media_assets (
+    id bigint NOT NULL,
+    created_at timestamp(6) without time zone NOT NULL,
+    updated_at timestamp(6) without time zone NOT NULL,
+    md5 character varying NOT NULL,
+    file_ext character varying NOT NULL,
+    file_size integer NOT NULL,
+    image_width integer NOT NULL,
+    image_height integer NOT NULL
+);
+
+
+--
+-- Name: media_assets_id_seq; Type: SEQUENCE; Schema: public; Owner: -
+--
+
+CREATE SEQUENCE public.media_assets_id_seq
+    START WITH 1
+    INCREMENT BY 1
+    NO MINVALUE
+    NO MAXVALUE
+    CACHE 1;
+
+
+--
+-- Name: media_assets_id_seq; Type: SEQUENCE OWNED BY; Schema: public; Owner: -
+--
+
+ALTER SEQUENCE public.media_assets_id_seq OWNED BY public.media_assets.id;
+
+
 --
 -- Name: mod_actions; Type: TABLE; Schema: public; Owner: -
 --
@@ -4279,6 +4314,13 @@ ALTER TABLE ONLY public.ip_bans ALTER COLUMN id SET DEFAULT nextval('public.ip_b
 ALTER TABLE ONLY public.ip_geolocations ALTER COLUMN id SET DEFAULT nextval('public.ip_geolocations_id_seq'::regclass);
 
 
+--
+-- Name: media_assets id; Type: DEFAULT; Schema: public; Owner: -
+--
+
+ALTER TABLE ONLY public.media_assets ALTER COLUMN id SET DEFAULT nextval('public.media_assets_id_seq'::regclass);
+
+
 --
 -- Name: mod_actions id; Type: DEFAULT; Schema: public; Owner: -
 --
@@ -4659,6 +4701,14 @@ ALTER TABLE ONLY public.ip_geolocations
     ADD CONSTRAINT ip_geolocations_pkey PRIMARY KEY (id);
 
 
+--
+-- Name: media_assets media_assets_pkey; Type: CONSTRAINT; Schema: public; Owner: -
+--
+
+ALTER TABLE ONLY public.media_assets
+    ADD CONSTRAINT media_assets_pkey PRIMARY KEY (id);
+
+
 --
 -- Name: mod_actions mod_actions_pkey; Type: CONSTRAINT; Schema: public; Owner: -
 --
@@ -6935,6 +6985,55 @@ CREATE INDEX index_ip_geolocations_on_time_zone ON public.ip_geolocations USING
 CREATE INDEX index_ip_geolocations_on_updated_at ON public.ip_geolocations USING btree (updated_at);
 
 
+--
+-- Name: index_media_assets_on_created_at; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_media_assets_on_created_at ON public.media_assets USING btree (created_at);
+
+
+--
+-- Name: index_media_assets_on_file_ext; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_media_assets_on_file_ext ON public.media_assets USING btree (file_ext);
+
+
+--
+-- Name: index_media_assets_on_file_size; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_media_assets_on_file_size ON public.media_assets USING btree (file_size);
+
+
+--
+-- Name: index_media_assets_on_image_height; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_media_assets_on_image_height ON public.media_assets USING btree (image_height);
+
+
+--
+-- Name: index_media_assets_on_image_width; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_media_assets_on_image_width ON public.media_assets USING btree (image_width);
+
+
+--
+-- Name: index_media_assets_on_md5; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_media_assets_on_md5 ON public.media_assets USING btree (md5);
+
+
+--
+-- Name: index_media_assets_on_updated_at; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_media_assets_on_updated_at ON public.media_assets USING btree (updated_at);
+
+
 --
 -- Name: index_mod_actions_on_created_at; Type: INDEX; Schema: public; Owner: -
 --
@@ -8016,5 +8115,6 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20210310221248'),
 ('20210330003356'),
 ('20210330093133');
+('20210901230931');