Add Web App support as 4th resource kind

[marcelloformica]

Summary

Adds Azure Web App (microsoft.web/sites) as the 4th supported resource kind alongside VMs, SQL Databases, and Storage Accounts.

Changes

Web App availability model

• MemoryWorkingSet metric (Average, bytes): >0 = available (1.0), =0 = stopped (suspect zero),
  ull = suspect null
• No degraded state — binary up/down model
• Function Apps excluded via kind contains 'functionapp' KQL filter

Activity Log lifecycle

• stop/start/restart operations with configurable grace window
• Paired stop→start spanning intervals — unlike VMs (where Resource Health reports 'Unavailable – Customer Initiated' for the entire deallocated period), stopped web apps show only 'Available' in Resource Health, so the full stopped window is inferred from Activity Log events

Universal improvements

• Creation/deletion state machine for all resource kinds (not just WebApp) — handles before-first-write, between delete→recreate cycles, and after-final-delete non-existence intervals
• MergeIntervals helper consolidates overlapping intervals

Bug fixes

• PS timestamp precision: \ConvertFrom-Json\ converts ISO 8601 strings to DateTime objects; [string]\ loses sub-second precision. Fixed by checking \ -is [datetime]\ and constructing DateTimeOffset directly
• C# locale formatting: SummaryWriter now uses \CultureInfo.InvariantCulture\ for decimal separators in summary output (was using system locale)

Cleanup

• Fixed duplicate XML summary in ActivityLogService
• Updated ResourceInventoryService doc to note Function App exclusion
• README already synced with all webapp additions

Verification

Both PS and C# versions tested against 3 subscriptions (231 resources, 850 output lines) for February 2026 — 0 data differences after fixes.