consolidate password_hash into 001 migration

Pre-1.0, no installed base — merge 002_add_passwords.sql into
001_initial.sql and remove the separate migration file.

internal/db/schema/001_initial.sql

@@ -1,11 +1,12 @@
 -- Chat server schema (pre-1.0 consolidated)
 PRAGMA foreign_keys = ON;
 
--- Sessions: IRC-style sessions (no passwords, nick + optional signing key)
+-- Sessions: each session is a user identity (nick + optional password + signing key)
 CREATE TABLE IF NOT EXISTS sessions (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     uuid TEXT NOT NULL UNIQUE,
     nick TEXT NOT NULL UNIQUE,
+    password_hash TEXT NOT NULL DEFAULT '',
     signing_key TEXT NOT NULL DEFAULT '',
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
     last_seen DATETIME DEFAULT CURRENT_TIMESTAMP

internal/db/schema/002_add_passwords.sql

@@ -1 +0,0 @@
-ALTER TABLE sessions ADD COLUMN password_hash TEXT NOT NULL DEFAULT '';