Merge template_usage_stats *_mins into app_usage_mins JSONB

Phase 4 of the Scalable Approach for Adding New IDE Session Types RFC. The durable template_usage_stats table still has dedicated vscode_mins/jetbrains_mins/ssh_mins/reconnecting_pty_mins columns, so adding an IDE would still require a schema change there; this phase makes the durable rollup layer dynamic too.

Scope (RFC: "template_usage_stats must also be migrated"):

• UpsertTemplateUsageStats: have the agent_stats_buckets CTE produce app_name/app_minutes entries that flow into the existing app_usage_mins JSONB, the same way web app usage is stored today
• Key scheme: builtin session names will share app_usage_mins with web app slugs; confirm how the insights queries tell them apart before writing the backfill
• One-time migration: backfill the dedicated *_mins values into app_usage_mins for existing rows, then drop the dedicated columns, including the always-zero sftp_mins (dead since its TODOs were never implementable)
• GetTemplateInsights: read IDE minutes from app_usage_mins instead of dedicated columns; convertTemplateInsightsApps builds the builtin apps list dynamically
• GetTemplateAppInsights stays separate (it filters via workspace_apps, which builtin IDE sessions never join)

After this phase, a new IDE flows agent → normalized table → JSONB rollup with zero schema changes, which is the point of the RFC.

Done when

• Rollup writes per-app and per-family minutes and the dedicated columns are dropped (feat: store session usage per app and family in template usage stats, child table template_usage_stats_session_apps instead of app_usage_mins, see the comment below on slug collisions)
• Backfilled historical data produces identical GetTemplateInsights results (feat: store session usage per app and family in template usage stats)
• Per-app read query over template_usage_stats_session_apps and dynamic convertTemplateInsightsApps
• A new app name reported through phase 1 shows up in template insights with no schema change
• Web app usage gets the same split: app_usage_mins broken out per app and returned from GetTemplateInsights alongside session usage (raised in review on feat: store session usage per app and family in template usage stats, since the child table means the two are now stored differently)

Depends on coder/coder#27410. Storage landed in feat: store session usage per app and family in template usage stats; per-app history starts at migration 000596.

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Created by Coder Agents on behalf of @EhabY.