feat: API response compression & payload optimization

[mendarb]

Summary

• Adds gzip/brotli compression via flask-compress middleware for all API responses
• Strips null fields from JSON payloads to reduce response size
• Adds ETag headers with If-None-Match support, returning 304 Not Modified when content hasn't changed
• Adds X-Original-Size / X-Compressed-Size response headers for measuring size reduction
• Adds pagination metadata (data + pagination envelope) to all list endpoints: /expenses, /bills, /categories, /reminders
• Includes a reusable paginate_query() helper and parse_pagination_args() utility

Closes #129

Changes

File	What
app/compression.py	New module: compression init, null stripping, ETag, pagination helpers
app/__init__.py	Wire in flask-compress and optimize_json_response after_request hook
app/routes/expenses.py	Paginated response envelope for GET /expenses
app/routes/bills.py	Paginated response envelope for GET /bills
app/routes/categories.py	Paginated response envelope for GET /categories
app/routes/reminders.py	Paginated response envelope for GET /reminders
requirements.txt	Add flask-compress==1.15
tests/test_compression.py	11 new tests for ETags, null stripping, gzip, pagination
Existing test files	Updated to use response["data"] instead of bare list

Test plan

• ETags present on JSON responses
• Conditional GET returns 304 when ETag matches
• Null fields stripped from JSON output
• X-Original-Size / X-Compressed-Size headers present
• gzip-compressed responses decode correctly
• All list endpoints return {data: [...], pagination: {...}} envelope
• Custom page / page_size params respected
• Existing tests updated and passing

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