fix: add missing buffer cleanup in watermark.py and rotate_flip.py (#353)

[Mohanapriya-sparks]

Fixes #353

Changes Made:

• Added buffer cleanup (output.close()) in watermark.py
• Added buffer cleanup (output.close()) in rotate_flip.py
• Added image cleanup (img_copy.close()) in rotate_flip.py
• Added finally blocks to ensure cleanup always happens

Files Changed:

• backend/blueprints/watermark.py
• backend/blueprints/rotate_flip.py

Expected Outcome:

Memory usage remains stable after repeated watermark and rotate/flip operations.

GSSoC'26

Contributing under GirlScript Summer of Code 2026

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