fix(review): address 6 reviewer findings from pullrequestreview-4064746778

- service/user.go: fix ChangePassword docstring (no old-password verification)
- service/user.go: RevokeSession now maps db.ErrNotFound→ErrNotFound and
  all other store errors→ErrInternal, preventing internal failures from
  masquerading as 404s
- plugin/loader.go: update scanPluginDirectory comment to reflect fail-fast
  behavior; fix Lstat comment wording
- db/queries/sqlite/events.sql: CAST COALESCE result to INTEGER so sqlc
  generates int64 instead of interface{}
- api/plugins_handler.go: log install error server-side and return sanitized
  structured JSON response instead of raw err.Error()
- .github/workflows/ci.yml: remove continue-on-error from tag build steps
  so tag boundary drift fails CI"

Agent-Logs-Url: https://github.com/J3vb/OwnCord/sessions/6635420c-af26-4cc0-9397-5e5b37887437

Co-authored-by: J3vb <192430104+J3vb@users.noreply.github.com>

Server/db/queries/sqlite/events.sql

@@ -3,7 +3,7 @@
 INSERT INTO events (seq, event_type, channel_id, payload) VALUES (?, ?, ?, ?);
 
 -- name: GetMaxEventSeq :one
-SELECT COALESCE(MAX(seq), 0) FROM events;
+SELECT CAST(COALESCE(MAX(seq), 0) AS INTEGER) AS max_seq FROM events;
 
 -- name: GetEventsSince :many
 SELECT seq, event_type, channel_id, payload, created_at