Replace string-matching error detection with typed SQLite errors (closes #39) (#66)

## Summary

Replaces fragile `strings.Contains(err.Error(), "UNIQUE")` checks with typed error detection using `errors.As` and the SQLite driver's `*sqlite.Error` type.

## Changes

- **`internal/db/errors.go`** (new): Adds `IsUniqueConstraintError(err)` helper that uses `errors.As` to unwrap the error into `*sqlite.Error` and checks for `SQLITE_CONSTRAINT_UNIQUE` (code 2067).
- **`internal/handlers/api.go`**: Replaces two `strings.Contains(err.Error(), "UNIQUE")` calls with `db.IsUniqueConstraintError(err)` — in `handleCreateSessionError` and `executeNickChange`.
- **`internal/handlers/auth.go`**: Replaces one `strings.Contains(err.Error(), "UNIQUE")` call with `db.IsUniqueConstraintError(err)` — in `handleRegisterError`.

## Why

String matching on error messages is fragile — if the SQLite driver changes its error message format, the detection silently breaks. Using `errors.As` with the driver's typed error and checking the specific SQLite error code is robust, idiomatic Go, and immune to message format changes.

closes #39

<!-- session: agent:sdlc-manager:subagent:3fb0b8e2-d635-4848-a5bd-131c5033cdb1 -->

Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #66
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/db/errors.go

@@ -0,0 +1,20 @@
+// Package db provides database access and migration management.
+package db
+
+import (
+	"errors"
+
+	"modernc.org/sqlite"
+	sqlite3 "modernc.org/sqlite/lib"
+)
+
+// IsUniqueConstraintError reports whether err is a SQLite
+// unique-constraint violation.
+func IsUniqueConstraintError(err error) bool {
+	var sqliteErr *sqlite.Error
+	if !errors.As(err, &sqliteErr) {
+		return false
+	}
+
+	return sqliteErr.Code() == sqlite3.SQLITE_CONSTRAINT_UNIQUE
+}

internal/handlers/api.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"time"
 
+	"git.eeqj.de/sneak/neoirc/internal/db"
 	"git.eeqj.de/sneak/neoirc/internal/irc"
 	"github.com/go-chi/chi"
 )
@@ -199,7 +200,7 @@ func (hdlr *Handlers) handleCreateSessionError(
 	request *http.Request,
 	err error,
 ) {
-	if strings.Contains(err.Error(), "UNIQUE") {
+	if db.IsUniqueConstraintError(err) {
 		hdlr.respondError(
 			writer, request,
 			"nick already taken",
@@ -1427,7 +1428,7 @@ func (hdlr *Handlers) executeNickChange(
 		request.Context(), sessionID, newNick,
 	)
 	if err != nil {
-		if strings.Contains(err.Error(), "UNIQUE") {
+		if db.IsUniqueConstraintError(err) {
 			hdlr.respondIRCError(
 				writer, request, clientID, sessionID,
 				irc.ErrNicknameInUse, nick, []string{newNick},

internal/handlers/auth.go

@@ -4,6 +4,8 @@ import (
 	"encoding/json"
 	"net/http"
 	"strings"
+
+	"git.eeqj.de/sneak/neoirc/internal/db"
 )
 
 const minPasswordLength = 8
@@ -94,7 +96,7 @@ func (hdlr *Handlers) handleRegisterError(
 	request *http.Request,
 	err error,
 ) {
-	if strings.Contains(err.Error(), "UNIQUE") {
+	if db.IsUniqueConstraintError(err) {
 		hdlr.respondError(
 			writer, request,
 			"nick already taken",