Added SQLite Web Worker Plan document

SQLITE_IMPL_PLAN.md

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+# SQLite Implementation Plan
+
+To avoid this work getting lost down the line, and to prevent progress from stalling, I'm writing down my ideas for SQLite implementation.
+
+## Plan
+
+_Web Workers have already been verified to work on both TSMM and r2modman. They have been added alongside Comlink to make usage easier._
+
+_The storage backend will use OPFS-SAH. Whilst restricted to singular read/write operations, our writes are in batches so should have negligible impact._
+_Promises handle switching between operations when necessary._
+
+### Step 1: Add the initial tables and migrations
+
+This is needed so that we get setup our expected format.
+
+We can optimize this significantly to what was previously stored in IndexedDB. We can properly use JOINs and indexes.
+
+### Step 2: Add upsert logic
+
+The pre-requirement data wise is to actually get that into the database.
+The upsert logic is needed so that we can update existing records, and insert new ones if they don't exist.
+
+### Step 2.5 (Benchmark times)
+
+We need to benchmark the upsert logic so that we can verify performance wins over IndexedDB.
+
+### Step 3: Feature flags
+
+Feature flags are needed so that we can switch between SQLite and IndexedDB implementations.
+It enables us to continuously merge code without the need to make breaking changes. Work doesn't have to stall and work can continue to be merged.
+
+The feature flags should be added so that we can:
+1. Disable SQLite entirely to prevent double insertion overhead in production builds
+2. Enable individual query actions to use SQLite once implemented
+
+Something along the lines of:
+```
+VITE_SQLITE_ENABLED=true
+VITE_SQLITE_SEARCH_ENABLED=true
+...
+```
+
+----
+
+This is a theoretical stopping point. SQLite is "implemented" (exists in the backend), but is not ready for production use.
+
+----
+
+### Step 4: Replace the search logic
+
+Let's immediately replace the search logic so that we can test performance and memory wins over IndexedDB.
+
+### Step 5: Replace remaining usages
+
+This is a large open task, but in reality, other usages are just specialized queries.
+
+### Step 6: Final testing and cleanup
+
+Remove the feature flags as this should be in a state ready to deploy to production. No remaining IndexedDB usages should exist for tsPackages.