fix: address code review findings

- Fix notifications_sent PK: changed from (rgid, sent_at) to rgid-only PK
  to prevent duplicate RGID rows across seconds. Use INSERT OR REPLACE
  instead of INSERT OR IGNORE for true idempotency.
- Add foreign key constraints to DDL (artist_id references artist_settings,
  rgid references external_releases) per specification.
- Enable PRAGMA foreign_keys=ON and PRAGMA busy_timeout=5000 for concurrent
  access safety.
- Fix GetNotificationSentAt query: add ORDER BY sent_at DESC LIMIT 1 for
  deterministic results.
- Fix config test: change YAML key from 'chat' to 'chat_id' to match struct
  tag, add ChatID assertion.
- Fix migration tracking test: correct error message from "expected 4" to
  "expected 3".
- Remove dead code in TestLoadConfig_InvalidPort: eliminate unused YAML
  template and remove port 0 case (valid, not invalid).
- Remove unused path parameter from buildConfigWithPort helper.
- Remove pointless 100ms sleep in run() and unused time import.
- Remove tautological TestDefaultConfigPath test.
- Update README.md: Go version 1.21+ to 1.25+, placeholder passwords to
  CHANGE_ME.
- Update config.yaml.example: placeholder passwords to CHANGE_ME.
- Update all database tests to insert parent rows first for FK satisfaction.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -28,7 +28,7 @@ NaviWatcher is an autonomous service daemon that monitors your Navidrome music c
 
 ### Technology Stack
 
-- **Language:** Go 1.21+
+- **Language:** Go 1.25+
 - **Database:** SQLite 3
 - **HTTP Server:** Go standard library (`net/http` + `html/template`)
 - **Key dependencies:**
@@ -69,12 +69,12 @@ server:
   host: "0.0.0.0"
   port: 8080
   username: "admin"
-  password: "password123"
+  password: "CHANGE_ME"
 
 navidrome:
   url: "http://localhost:4533"
   user: "watcher_service"
-  password: "user_password"
+  password: "CHANGE_ME"
 
 musicbrainz:
   user_agent: "NaviWatcher/1.0 ( mail@example.com )"
@@ -176,12 +176,12 @@ server:
   host: "0.0.0.0"
   port: 8080
   username: "admin"
-  password: "password123"
+  password: "CHANGE_ME"
 
 navidrome:
   url: "http://localhost:4533"
   user: "watcher_service"
-  password: "user_password"
+  password: "CHANGE_ME"
 
 musicbrainz:
   user_agent: "NaviWatcher/1.0 ( mail@example.com )"