Validate upload content before persisting to /uploads

[ORDL-AMF]

Motivation

• Close a stored-XSS/content-hosting hole where uploaded bytes were saved with attacker-controlled extensions and exposed at /uploads without content validation.
• Ensure only bona fide raster images are persisted so the static /uploads mount cannot serve HTML/SVG/JS from user uploads.

Description

• Replaced extension-from-filename/content-type logic with a content-based validator by adding _validated_image_suffix(content: bytes) which attempts to parse the bytes with Pillow and maps trusted raster formats to safe extensions (.jpg, .png, .webp, .gif, .bmp, .tiff).
• Updated _persist_uploaded_asset to call _validated_image_suffix and only write files when the content is a supported image; otherwise it returns None and logs a warning, preserving existing behavior of returning asset_url when persisted.
• Added imports required for validation (BytesIO, PIL.Image, UnidentifiedImageError) and removed the prior filename-suffix reliance.

Testing

• Ran python -m py_compile aether-system/backend/api/routes.py and it succeeded to validate syntax.
• Attempted a small runtime validation of _validated_image_suffix but the environment lacked fastapi which caused import failure (ModuleNotFoundError: No module named 'fastapi'), so dynamic execution tests were not completed.
• Committed the change after static checks and ensured the modified module compiles cleanly.

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Codex Task