feat: store auth tokens as SHA-256 hashes instead of plaintext (#69)

## Summary

Hash client auth tokens with SHA-256 before storing in the database. When validating tokens, hash the incoming token and compare against the stored hash. This prevents token exposure if the database is compromised.

Existing plaintext tokens are implicitly invalidated since they will not match the new hashed lookups — users will need to create new sessions.

## Changes

- **`internal/db/queries.go`**: Added `hashToken()` helper using `crypto/sha256`. Updated `CreateSession` to store hashed token. Updated `GetSessionByToken` to hash the incoming token before querying.
- **`internal/db/auth.go`**: Updated `RegisterUser` and `LoginUser` to store hashed tokens.

## Migration

No schema changes needed. The `token` column remains `TEXT` but now stores 64-char hex SHA-256 digests instead of 64-char hex random tokens. Existing plaintext tokens are effectively invalidated.

closes #34

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #69
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/db/auth.go

@@ -64,12 +64,14 @@ func (database *Database) RegisterUser(
 
 	sessionID, _ := res.LastInsertId()
 
+	tokenHash := hashToken(token)
+
 	clientRes, err := transaction.ExecContext(ctx,
 		`INSERT INTO clients
 		 (uuid, session_id, token,
 		  created_at, last_seen)
 		 VALUES (?, ?, ?, ?, ?)`,
-		clientUUID, sessionID, token, now, now)
+		clientUUID, sessionID, tokenHash, now, now)
 	if err != nil {
 		_ = transaction.Rollback()
 
@@ -137,12 +139,14 @@ func (database *Database) LoginUser(
 
 	now := time.Now()
 
+	tokenHash := hashToken(token)
+
 	res, err := database.conn.ExecContext(ctx,
 		`INSERT INTO clients
 		 (uuid, session_id, token,
 		  created_at, last_seen)
 		 VALUES (?, ?, ?, ?, ?)`,
-		clientUUID, sessionID, token, now, now)
+		clientUUID, sessionID, tokenHash, now, now)
 	if err != nil {
 		return 0, 0, "", fmt.Errorf(
 			"create login client: %w", err,

internal/db/queries.go

@@ -3,6 +3,7 @@ package db
 import (
 	"context"
 	"crypto/rand"
+	"crypto/sha256"
 	"database/sql"
 	"encoding/hex"
 	"encoding/json"
@@ -31,6 +32,14 @@ func generateToken() (string, error) {
 	return hex.EncodeToString(buf), nil
 }
 
+// hashToken returns the lowercase hex-encoded SHA-256
+// digest of a plaintext token string.
+func hashToken(token string) string {
+	sum := sha256.Sum256([]byte(token))
+
+	return hex.EncodeToString(sum[:])
+}
+
 // IRCMessage is the IRC envelope for all messages.
 type IRCMessage struct {
 	ID      string          `json:"id"`
@@ -105,12 +114,14 @@ func (database *Database) CreateSession(
 
 	sessionID, _ := res.LastInsertId()
 
+	tokenHash := hashToken(token)
+
 	clientRes, err := transaction.ExecContext(ctx,
 		`INSERT INTO clients
 		 (uuid, session_id, token,
 		  created_at, last_seen)
 		 VALUES (?, ?, ?, ?, ?)`,
-		clientUUID, sessionID, token, now, now)
+		clientUUID, sessionID, tokenHash, now, now)
 	if err != nil {
 		_ = transaction.Rollback()
 
@@ -143,6 +154,8 @@ func (database *Database) GetSessionByToken(
 		nick      string
 	)
 
+	tokenHash := hashToken(token)
+
 	err := database.conn.QueryRowContext(
 		ctx,
 		`SELECT s.id, c.id, s.nick
@@ -150,7 +163,7 @@ func (database *Database) GetSessionByToken(
 		 INNER JOIN sessions s
 		   ON s.id = c.session_id
 		 WHERE c.token = ?`,
-		token,
+		tokenHash,
 	).Scan(&sessionID, &clientID, &nick)
 	if err != nil {
 		return 0, 0, "", fmt.Errorf(