feat: add file attachment support to issue tracker with Gitea mirroring

Implement full attachment CRUD for issues and comments, restricted to open issues only. Attachments are stored locally and mirrored to the Gitea tracker via new `mirror_attachment_to_gitea` and `set_gitea_asset_id` functions. Add `gitea_asset_id` column to the attachments table, extend `IssueForm` and `IssueCommentForm` with `attachment_uids` field, and expose new API endpoints for listing, adding, and deleting issue attachments. Update agent configuration to include web research tools, and document the new capability in README, AGENTS.md, and CLAUDE.md.

README.md

@@ -82,7 +82,7 @@ devplacepy/
 | `/mute` | Mute/unmute a user: stops them creating notifications for you while their content stays visible |
 | `/leaderboard` | Contributor ranking by total stars earned |
 | `/avatar` | Multiavatar proxy with in-memory cache |
-| `/issues` | Issue tracker backed by Gitea: list/detail/comment/status, AI-enhanced filing, admin planning report of all open tickets |
+| `/issues` | Issue tracker backed by Gitea: list/detail/comment/status, AI-enhanced filing, admin planning report of all open tickets, and file attachments on open issues and comments (mirrored to the Gitea tracker) |
 | `/admin/services` | Background service management (start/stop, config, status, logs) |
 | `/admin/bots` | Admin **Bot Monitor**: a live grid of the latest low-quality screenshot per running bot persona, each labelled with the bot username, persona, and current action, auto-refreshing |
 | `/admin` | Admin panel (user management, news curation, settings) |