Centralize dashboard request and response helpers

[raychrisgdp]

Goal

Centralize small shared request/response helpers currently living in app.py so route modules can stay focused.

Candidate Scope

• Range parsing helpers such as parse_days and parse_top_n
• Simulated-mode detection
• API response helpers such as empty daily responses
• Lightweight profiling helper if still needed after route extraction

Constraints

• Do not create broad utility modules with unrelated helpers.
• Keep helper names explicit and tied to dashboard API behavior.
• Add or update focused tests if behavior is moved.

Verification

• Unit tests pass before and after the move.
• No API response contract changes.