Refactor to apps/ structure

JIGAIDO is now a platform with apps/ as the container.
All telegram-bot files moved to apps/telegram-bot/:
- bot.py, commands.py, cron.py, db.py, schema.sql
- requirements.txt, .env.example, README.md
- Root now holds SPEC.md, README.md, CONTRIBUTING.md only.

Structure:
jigaido/
├── apps/
│   └── telegram-bot/
└── SPEC.md, README.md, CONTRIBUTING.md

apps/telegram-bot/schema.sql

@@ -0,0 +1,49 @@
+-- JIGAIDO Database Schema
+
+CREATE TABLE IF NOT EXISTS groups (
+    id              INTEGER PRIMARY KEY AUTOINCREMENT,
+    telegram_chat_id INTEGER UNIQUE NOT NULL,
+    creator_user_id  INTEGER NOT NULL,
+    created_at       INTEGER NOT NULL DEFAULT (unixepoch())
+);
+
+CREATE TABLE IF NOT EXISTS group_admins (
+    group_id  INTEGER REFERENCES groups(id) ON DELETE CASCADE,
+    user_id   INTEGER NOT NULL,
+    PRIMARY KEY (group_id, user_id)
+);
+
+CREATE TABLE IF NOT EXISTS users (
+    id               INTEGER PRIMARY KEY AUTOINCREMENT,
+    telegram_user_id  INTEGER UNIQUE NOT NULL,
+    username          TEXT,
+    created_at        INTEGER NOT NULL DEFAULT (unixepoch())
+);
+
+CREATE TABLE IF NOT EXISTS bounties (
+    id                   INTEGER PRIMARY KEY AUTOINCREMENT,
+    group_id              INTEGER REFERENCES groups(id) ON DELETE CASCADE,
+    created_by_user_id    INTEGER REFERENCES users(id),
+    informed_by_username  TEXT NOT NULL,
+    text                  TEXT,
+    link                  TEXT,
+    due_date_ts           INTEGER,
+    created_at            INTEGER NOT NULL DEFAULT (unixepoch()),
+    UNIQUE(group_id, link)
+);
+
+CREATE TABLE IF NOT EXISTS user_bounty_tracking (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id    INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    bounty_id  INTEGER NOT NULL REFERENCES bounties(id) ON DELETE CASCADE,
+    added_at   INTEGER NOT NULL DEFAULT (unixepoch()),
+    UNIQUE(user_id, bounty_id)
+);
+
+CREATE TABLE IF NOT EXISTS reminder_log (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id     INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    bounty_id   INTEGER NOT NULL REFERENCES bounties(id) ON DELETE CASCADE,
+    reminded_at INTEGER NOT NULL DEFAULT (unixepoch()),
+    UNIQUE(user_id, bounty_id)
+);