feat(friends): Start implementing database layer

DB/friendships.sql

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+CREATE TYPE friendship_status AS ENUM ('pending', 'accepted', 'declined', 'blocked');
+
+CREATE TABLE friendships
+(
+    friendship_id  BIGSERIAL PRIMARY KEY,
+    user1_id       UUID                     NOT NULL,
+    user2_id       UUID                     NOT NULL,
+    status         friendship_status        NOT NULL,
+    action_user_id UUID                     NOT NULL, -- The user who performed the last action
+    created_at     TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT now(),
+    updated_at     TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT now(),
+
+    -- Ensure user1_id is always less than user2_id to prevent duplicate friendships
+    CONSTRAINT ensure_user_order CHECK (user1_id < user2_id),
+    CONSTRAINT unique_friendship UNIQUE (user1_id, user2_id),
+
+    -- Foreign keys
+    CONSTRAINT fk_user1 FOREIGN KEY (user1_id) REFERENCES users (id),
+    CONSTRAINT fk_user2 FOREIGN KEY (user2_id) REFERENCES users (id),
+    CONSTRAINT fk_action_user FOREIGN KEY (action_user_id) REFERENCES users (id)
+);
+
+CREATE INDEX idx_friendship_user1 ON friendships (user1_id, status);
+CREATE INDEX idx_friendship_user2 ON friendships (user2_id, status);
+
+-- Functions
+
+-- automatically update updated_at timestamp
+CREATE OR REPLACE FUNCTION update_updated_at_column()
+    RETURNS TRIGGER AS
+$$
+BEGIN
+    NEW.updated_at = now();
+    RETURN NEW;
+END;
+$$;
+
+CREATE TRIGGER update_friendships_timestamp
+    BEFORE UPDATE
+    ON friendships
+    FOR EACH ROW
+EXECUTE FUNCTION update_updated_at_column();
+
+-- Send a friend request
+CREATE OR REPLACE FUNCTION send_friend_request(sender_id UUID, receiver_id UUID) RETURNS void AS
+$$
+DECLARE
+    smaller_id UUID;
+    larger_id  UUID;
+BEGIN
+    -- Determine order of IDs using UUID comparison
+    IF sender_id < receiver_id THEN
+        smaller_id := sender_id;
+        larger_id := receiver_id;
+    ELSE
+        smaller_id := receiver_id;
+        larger_id := sender_id;
+    END IF;
+
+    -- Insert friendship record
+    INSERT INTO friendships (user1_id, user2_id, status, action_user_id)
+    VALUES (smaller_id, larger_id, 'pending', sender_id)
+    ON CONFLICT (user1_id, user2_id) DO UPDATE
+        SET status         = CASE
+                                 WHEN friendships.status = 'declined' THEN 'pending'::friendship_status
+                                 ELSE friendships.status
+            END,
+            action_user_id = sender_id;
+END;
+$$ LANGUAGE plpgsql;
+
+-- retrieve incoming friend requests
+CREATE OR REPLACE FUNCTION get_incoming_friend_requests(user_id UUID)
+    RETURNS TABLE
+            (
+                friendship_id BIGINT,
+                sender_id     UUID,
+                receiver_id   UUID,
+                status        friendship_status,
+                created_at    TIMESTAMP WITH TIME ZONE,
+                updated_at    TIMESTAMP WITH TIME ZONE
+            )
+AS
+$$
+BEGIN
+    RETURN QUERY
+        SELECT f.friendship_id,
+               CASE
+                   WHEN f.user1_id = user_id THEN f.user2_id
+                   ELSE f.user1_id
+                   END AS sender_id,
+               user_id AS receiver_id,
+               f.status,
+               f.created_at,
+               f.updated_at
+        FROM friendships f
+        WHERE (f.user1_id = user_id OR f.user2_id = user_id)
+          AND f.status = 'pending'
+          AND f.action_user_id != user_id;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE OR REPLACE FUNCTION accept_friend_request_by_id(friendship_id_param BIGINT)
+    RETURNS void AS
+$$
+BEGIN
+    UPDATE friendships
+    SET status = 'accepted'
+    WHERE friendship_id = friendship_id_param
+      AND status = 'pending';
+
+    IF NOT FOUND THEN
+        RAISE EXCEPTION 'No pending friend request found with this ID';
+    END IF;
+END;
+$$ LANGUAGE plpgsql;
+
+
+-- examples
+SELECT *
+FROM get_incoming_friend_requests('d2e6a9a3-a0be-45ce-ae39-676c6a88c53a');
+SELECT accept_friend_request_by_id(4::bigint);

DB/users.sql

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+CREATE TABLE users
+(
+    id                 uuid    not null references auth.users on delete cascade,
+    avatar_url         text,
+    username           text    not null,
+    spotify_id         text    not null,
+    spotify_visibility boolean not null default false,
+    primary key (id)
+);
+
+ALTER TABLE users
+    ADD CONSTRAINT unique_username UNIQUE (username),
+    ADD CONSTRAINT valid_username check (username <> '' AND length(trim(username)) >= 4 AND username ~ '^[a-zA-Z0-9_]+$');
+
+CREATE INDEX idx_username ON users(username);

server/api/v1/user/friends/index.delete.ts

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+// remove friend

server/api/v1/user/friends/index.get.ts

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+// Not relative to userid because you should always only be able to see your own friends

server/api/v1/user/friends/index.post.ts

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+// send friend invite

server/api/v1/user/friends/index.put.ts

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+// accept or decline friend request