Fix cache data leak, decimal inaccuracies, and timezone boundaries

[sam-aydev]

• Fixed cross-company data leak: Added tenant_id to the Redis cache key in services/cache.py to ensure isolated caching per client.
• Fixed cent inaccuracies: Replaced float with Decimal in api/v1/dashboard.py to prevent floating-point arithmetic errors in financial totals.
• Fixed March revenue mismatch: Added tzinfo=timezone.utc to datetime bounds in services/reservations.py to prevent local server time from shifting revenues into the wrong month.

[sentry]

Bug: Returning a Decimal for total_revenue causes FastAPI under Pydantic v2 to serialize it as a string, violating the frontend numeric contract.
_{Severity: LOW}

Suggested Fix

Cast total_revenue to float before returning it in the API response, or define an explicit Pydantic response model configuring decimal serialization as a float.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: backend/app/api/v1/dashboard.py#L19-L25

Potential issue: Returning a `Decimal` instance for `total_revenue` in an endpoint
returning `Dict[str, Any]` causes FastAPI under Pydantic v2 to serialize the value as a
JSON string instead of a number. The frontend TypeScript types define `total_revenue` as
a `number`, breaking the API contract and causing numeric values to be displayed as
quoted strings in raw data views.

_{Did we get this right? 👍 / 👎 to inform future reviews.}