Created database for user interests.

Backend.cs

@@ -10,6 +10,7 @@ builder.Services.AddSwaggerGen();
 
 builder.Services.AddSingleton<IHistoricalMarketDataCache, PostgresHistoricalMarketDataCache>();
 builder.Services.AddSingleton<MarketDataService>();
+builder.Services.AddSingleton<IUserInterestStore, PostgresUserInterestStore>();
 
 builder.Services.AddCors(options =>
 {

Controller/UserInterestController.cs

@@ -0,0 +1,66 @@
+using Backend.DatabaseHandler;
+using Backend.UserInterests;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Backend.Controllers;
+
+[ApiController]
+[Route("api/user-interest")]
+public sealed class UserInterestController : ControllerBase
+{
+	private readonly IUserInterestStore _store;
+
+	public UserInterestController(IUserInterestStore store)
+	{
+		_store = store;
+	}
+
+	[HttpGet("fetch")]
+	public async Task<ActionResult<UserInterestFetchResponse>> FetchAll(
+		[FromQuery] string user,
+		[FromQuery] string type)
+	{
+		try
+		{
+			List<string> items = await _store.FetchAllAsync(user, type);
+			return Ok(new UserInterestFetchResponse
+			{
+				Items = items
+			});
+		}
+		catch (ArgumentException ex)
+		{
+			return BadRequest(ex.Message);
+		}
+	}
+
+	[HttpPost("add")]
+	public async Task<ActionResult<BoolResponse>> Add(
+		[FromBody] UserInterestModifyRequest request)
+	{
+		try
+		{
+			bool success = await _store.AddAsync(request.User, request.Type, request.Value);
+			return Ok(new BoolResponse { Success = success });
+		}
+		catch (ArgumentException ex)
+		{
+			return BadRequest(ex.Message);
+		}
+	}
+
+	[HttpDelete("remove")]
+	public async Task<ActionResult<BoolResponse>> Remove(
+		[FromBody] UserInterestModifyRequest request)
+	{
+		try
+		{
+			bool success = await _store.RemoveAsync(request.User, request.Type, request.Value);
+			return Ok(new BoolResponse { Success = success });
+		}
+		catch (ArgumentException ex)
+		{
+			return BadRequest(ex.Message);
+		}
+	}
+}

DatabaseHandler/IUserInterestCache.cs

@@ -0,0 +1,8 @@
+namespace Backend.DatabaseHandler;
+
+public interface IUserInterestStore
+{
+	Task<List<string>> FetchAllAsync(string user, string type);
+	Task<bool> AddAsync(string user, string type, string value);
+	Task<bool> RemoveAsync(string user, string type, string value);
+}

DatabaseHandler/PostgresUserInterestStore.cs

@@ -0,0 +1,141 @@
+using Npgsql;
+
+namespace Backend.DatabaseHandler;
+
+public sealed class PostgresUserInterestStore : IUserInterestStore
+{
+	private readonly string _connectionString;
+
+	public PostgresUserInterestStore(IConfiguration configuration)
+	{
+		_connectionString = configuration.GetConnectionString("MarketDataDb")
+			?? throw new InvalidOperationException("Missing connection string: MarketDataDb");
+	}
+
+	public async Task<List<string>> FetchAllAsync(string user, string type)
+	{
+		ValidateUser(user);
+
+		string normalizedType = NormalizeType(type);
+		string sql = normalizedType == "symbol"
+			? """
+              SELECT symbol
+              FROM user_interest_symbols
+              WHERE username = @user
+              ORDER BY symbol ASC;
+              """
+			: """
+              SELECT topic
+              FROM user_interest_topics
+              WHERE username = @user
+              ORDER BY topic ASC;
+              """;
+
+		var results = new List<string>();
+
+		await using var conn = new NpgsqlConnection(_connectionString);
+		await conn.OpenAsync();
+
+		await using var cmd = new NpgsqlCommand(sql, conn);
+		cmd.Parameters.AddWithValue("user", user.Trim());
+
+		await using var reader = await cmd.ExecuteReaderAsync();
+		while (await reader.ReadAsync())
+		{
+			results.Add(reader.GetString(0));
+		}
+
+		return results;
+	}
+
+	public async Task<bool> AddAsync(string user, string type, string value)
+	{
+		ValidateUser(user);
+		ValidateValue(value);
+
+		string normalizedType = NormalizeType(type);
+		string normalizedValue = normalizedType == "symbol"
+			? value.Trim().ToUpperInvariant()
+			: value.Trim();
+
+		string sql = normalizedType == "symbol"
+			? """
+              INSERT INTO user_interest_symbols (username, symbol)
+              VALUES (@user, @value)
+              ON CONFLICT (username, symbol) DO NOTHING;
+              """
+			: """
+              INSERT INTO user_interest_topics (username, topic)
+              VALUES (@user, @value)
+              ON CONFLICT (username, topic) DO NOTHING;
+              """;
+
+		await using var conn = new NpgsqlConnection(_connectionString);
+		await conn.OpenAsync();
+
+		await using var cmd = new NpgsqlCommand(sql, conn);
+		cmd.Parameters.AddWithValue("user", user.Trim());
+		cmd.Parameters.AddWithValue("value", normalizedValue);
+
+		int rows = await cmd.ExecuteNonQueryAsync();
+		return rows > 0;
+	}
+
+	public async Task<bool> RemoveAsync(string user, string type, string value)
+	{
+		ValidateUser(user);
+		ValidateValue(value);
+
+		string normalizedType = NormalizeType(type);
+		string normalizedValue = normalizedType == "symbol"
+			? value.Trim().ToUpperInvariant()
+			: value.Trim();
+
+		string sql = normalizedType == "symbol"
+			? """
+              DELETE FROM user_interest_symbols
+              WHERE username = @user AND symbol = @value;
+              """
+			: """
+              DELETE FROM user_interest_topics
+              WHERE username = @user AND topic = @value;
+              """;
+
+		await using var conn = new NpgsqlConnection(_connectionString);
+		await conn.OpenAsync();
+
+		await using var cmd = new NpgsqlCommand(sql, conn);
+		cmd.Parameters.AddWithValue("user", user.Trim());
+		cmd.Parameters.AddWithValue("value", normalizedValue);
+
+		int rows = await cmd.ExecuteNonQueryAsync();
+		return rows > 0;
+	}
+
+	private static string NormalizeType(string type)
+	{
+		string normalized = type.Trim().ToLowerInvariant();
+		return normalized switch
+		{
+			"symbol" => "symbol",
+			"topic" => "topic",
+			_ => throw new ArgumentException("Type must be either 'symbol' or 'topic'.")
+		};
+	}
+
+	private static void ValidateUser(string user)
+	{
+		if (string.IsNullOrWhiteSpace(user))
+		{
+			throw new ArgumentException("User cannot be empty.");
+		}
+	}
+
+	private static void ValidateValue(string value)
+	{
+		if (string.IsNullOrWhiteSpace(value))
+		{
+			throw new ArgumentException("Value cannot be empty.");
+		}
+	}
+}

UserInterests/UserInterests.cs

@@ -0,0 +1,24 @@
+namespace Backend.UserInterests;
+
+public sealed class UserInterestFetchRequest
+{
+	public string User { get; set; } = string.Empty;
+	public string Type { get; set; } = string.Empty; // "symbol" or "topic"
+}
+
+public sealed class UserInterestModifyRequest
+{
+	public string User { get; set; } = string.Empty;
+	public string Type { get; set; } = string.Empty; // "symbol" or "topic"
+	public string Value { get; set; } = string.Empty; // symbol or topic
+}
+
+public sealed class UserInterestFetchResponse
+{
+	public List<string> Items { get; set; } = new();
+}
+
+public sealed class BoolResponse
+{
+	public bool Success { get; set; }
+}