feat(dimensional): add dim_time for intraday time-of-day analysis

[blarghmatey]

Context

PR #1868 established the core dimensional model schema including dim_date. Reviewer requested a time-of-day dimension to support intraday analytics.

What

Add a dim_time dimension table to complement dim_date for sub-daily analysis:

• Grain: one row per minute (or second) of the day — 1,440 rows for minute-grain
• Columns: time_key (integer HHMM or HHMMSS), hour, minute, second, am_pm, hour_of_day_label, time_of_day_bucket (morning/afternoon/evening/night), etc.
• Fact tables with event timestamps (e.g., tfact_video_events, tfact_problem_events, tfact_chatbot_events) should gain a time_fk column

Acceptance Criteria

• dim_time model created in src/ol_dbt/models/dimensional/
• _dim_time.yml with PK uniqueness test and column documentation
• time_fk added to relevant event fact tables
• dbt test: PK unique, not_null, FK relationships valid