db+adapter, add sqlx + schema & add adapter

Adds SQLX for database management, and an Adapter interface for
interacting with it. Through the type "AdapterPool"

Creates an initial_schema with everything I think I'll need.

koucha/migrations/20260115003047_initial_schema.sql

@@ -0,0 +1,62 @@
+-- Add migration script here
+
+PRAGMA foreign_keys = ON;
+
+CREATE TABLE users (
+  id INTEGER PRIMARY KEY,
+  name TEXT UNIQUE NOT NULL
+);
+
+CREATE TABLE channels (
+  id INTEGER PRIMARY KEY,
+  title TEXT NOT NULL,
+  link TEXT UNIQUE NOT NULL,
+  description TEXT,
+  last_fetched TEXT
+);
+
+CREATE TABLE items (
+  id INTEGER PRIMARY KEY,
+  channel_id INTEGER NOT NULL,
+  guid TEXT NOT NULL,
+
+  fetched_at TEXT,
+  title TEXT,
+  description TEXT,
+  content TEXT,
+
+  UNIQUE(channel_id, guid),
+  FOREIGN KEY (channel_id) REFERENCES channels(id)
+);
+
+CREATE TABLE feeds (
+  id INTEGER PRIMARY KEY,
+  user_id INTEGER NOT NULL,
+  title TEXT NOT NULL,
+  FOREIGN KEY (user_id) REFERENCES users(id)
+);
+
+CREATE TABLE feed_channels (
+  feed_id INTEGER NOT NULL,
+  channel_id INTEGER NOT NULL,
+  initial_score INTEGER,
+  gravity INTEGER,
+  boost INTEGER,
+  PRIMARY KEY (feed_id, channel_id),
+  FOREIGN KEY (feed_id) REFERENCES feeds(id),
+  FOREIGN KEY (channel_id) REFERENCES channels(id)
+);
+
+CREATE TABLE feed_items (
+  item_id INTEGER NOT NULL,
+  feed_id INTEGER NOT NULL,
+  score INTEGER NOT NULL,
+  last_updated TEXT NOT NULL,
+  boosted_at TEXT,
+  archived BOOLEAN DEFAULT FALSE,
+  PRIMARY KEY (item_id, feed_id),
+  FOREIGN KEY (feed_id) REFERENCES feeds(id),
+  FOREIGN KEY (item_id) REFERENCES items(id)
+);
+CREATE INDEX idx_feed_items_score
+ON feed_items(feed_id, archived, score DESC);