feat(files): persist uploaded files and associate them with conversations (#42)

[Hustler490]

Closes #42.

What

Persists uploaded files on disk and records them in a new files table so they can be listed, downloaded, attached to a conversation, and re-analyzed later.

Model + schema

• New UploadedFile model (app/models/uploaded_file.py, table files): id, workspace_id, user_id, original_name, stored_name, size, content_type, created_at plus an optional conversation_id association.
• Alembic migration e2d3c4b5a6f7_add_files_table.py (upgrade creates the table + indexes; downgrade drops it). tests/test_migrations.py head assertion updated to the new revision.

Storage + validation (app/services/uploads.py)

• Files are written under the configurable UPLOAD_FOLDER (app/config.py; a throwaway temp dir in testing) with a random stored_name.
• Extension, UTF-8, and size checks reuse the /tools/analyze allowlist and 200k-char cap (ALLOWED_EXTENSIONS/MAX_UPLOAD_CHARS are now defined once and shared).

Routes (/tools/files)

• POST /tools/files upload, GET /tools/files list, GET /tools/files/<id> metadata, GET /tools/files/<id>/download, POST /tools/files/<id>/attach (associate with a conversation), POST /tools/files/<id>/reanalyze. All are owner-scoped (user_id == current_user.id) and 404 for other users.

Tests (tests/test_files_persistence.py)

Upload (metadata + on-disk bytes), retrieval, download, attach, re-analyze, unsupported-type rejection, and owner-only access.

Acceptance criteria

• New files table with the requested columns.
• Uploads stored under a configurable UPLOAD_FOLDER with random names and validated extensions.
• Files can be attached to a conversation and re-analyzed later.
• Size/type checks reuse /tools/analyze validation.
• Tests cover upload, retrieval, download, and owner-only access.

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