Writeup backend database specification

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+# Backend Database Specification
+
+The backend will use sqlite to store all state for storing players (and their
+IDs, and paired state), and games in progress.
+
+## Table definitions
+
+```
+// enable foreign_keys restrictions
+PRAGMA foreign_keys = ON;
+
+// fields should be self explanatory for the players table
+CREATE TABLE players (id INTEGER PRIMARY KEY NOT NULL,
+                      date_added TEXT NOT NULL,
+                      game_id INTEGER,
+                      FOREIGN KEY(game_id) REFERENCES games(id));
+
+// "cyan_player" and "magenta_player" should correspond to an existing entry in
+// table "players".
+
+// "board" is as explained in backend_protocol_specification.md
+
+// "status" is "0" for cyan's turn, "1" for magenta's turn, "2" for cyan won,
+// "3" for magenta won, "4" for draw.
+
+CREATE TABLE games (id INTEGER PRIMARY KEY ASC AUTOINCREMENT NOT NULL,
+                    cyan_player INTEGER UNIQUE NOT NULL,
+                    magenta_player INTEGER UNIQUE NOT NULL,
+                    date_changed TEXT NOT NULL,
+                    board TEXT NOT NULL,
+                    status INTEGER NOT NULL,
+                    FOREIGN KEY(cyan_player) REFERENCES players (id),
+                    FOREIGN KEY(magenta_player) REFERENCES players (id));
+```
+
+"date" entries are used for garbage collection of the database. A predefined
+length of time will be used to cleanup stale entries. Whenever an entry in the
+"games" table is updated, the "date" entry will be updated as well.