Wrap user create/delete flows in a Postgres transaction

Signup: post-user-insert + session-insert + CREATE MATERIALIZED VIEW
all ran un-transacted. Any failure between them (DB blip, view
collision from a prior failed attempt, container kill, max_connections,
etc.) left the users and session_ids rows orphaned and the materialized
view missing. Re-registering the same email then 500'd on the
view-create collision. Logged-in users hit 500 on /feed/subscriptions
because the view lookup referenced a non-existent relation.

Same pattern in account-delete (Users.delete + SessionIDs.delete +
DROP MATERIALIZED VIEW).

Fix: add an optional `conn` parameter (DB::Database | DB::Connection |
DB::Transaction, default PG_DB) to Users.insert/delete and all four
SessionIDs overloads, then wrap the create/delete call sites in
PG_DB.transaction { |tx| ... }. Default arg keeps every existing
call site working unchanged.

Verified on a fresh local stack:
- registration of a clean user: user + session + view all created
- registration with a pre-existing view: 500, zero rows in users and
  session_ids (rollback worked), pre-existing view untouched

Closes iv-org/invidious#2509

src/invidious/database/sessions.cr

@@ -7,7 +7,7 @@ module Invidious::Database::SessionIDs
   #  Insert
   # -------------------
 
-  def insert(sid : String, email : String, handle_conflicts : Bool = false)
+  def insert(sid : String, email : String, handle_conflicts : Bool = false, *, conn : DB::Database | DB::Connection | DB::Transaction = PG_DB)
     request = <<-SQL
       INSERT INTO session_ids
       VALUES ($1, $2, now())
@@ -15,38 +15,38 @@ module Invidious::Database::SessionIDs
 
     request += " ON CONFLICT (id) DO NOTHING" if handle_conflicts
 
-    PG_DB.exec(request, sid, email)
+    conn.exec(request, sid, email)
   end
 
   # -------------------
   #  Delete
   # -------------------
 
-  def delete(*, sid : String)
+  def delete(*, sid : String, conn : DB::Database | DB::Connection | DB::Transaction = PG_DB)
     request = <<-SQL
       DELETE FROM session_ids *
       WHERE id = $1
     SQL
 
-    PG_DB.exec(request, sid)
+    conn.exec(request, sid)
   end
 
-  def delete(*, email : String)
+  def delete(*, email : String, conn : DB::Database | DB::Connection | DB::Transaction = PG_DB)
     request = <<-SQL
       DELETE FROM session_ids *
       WHERE email = $1
     SQL
 
-    PG_DB.exec(request, email)
+    conn.exec(request, email)
   end
 
-  def delete(*, sid : String, email : String)
+  def delete(*, sid : String, email : String, conn : DB::Database | DB::Connection | DB::Transaction = PG_DB)
     request = <<-SQL
       DELETE FROM session_ids *
       WHERE id = $1 AND email = $2
     SQL
 
-    PG_DB.exec(request, sid, email)
+    conn.exec(request, sid, email)
   end
 
   # -------------------

src/invidious/database/users.cr

@@ -7,7 +7,7 @@ module Invidious::Database::Users
   #  Insert / delete
   # -------------------
 
-  def insert(user : User, update_on_conflict : Bool = false)
+  def insert(user : User, update_on_conflict : Bool = false, *, conn : DB::Database | DB::Connection | DB::Transaction = PG_DB)
     user_array = user.to_a
     user_array[4] = user_array[4].to_json # User preferences
 
@@ -23,16 +23,16 @@ module Invidious::Database::Users
       SQL
     end
 
-    PG_DB.exec(request, args: user_array)
+    conn.exec(request, args: user_array)
   end
 
-  def delete(user : User)
+  def delete(user : User, *, conn : DB::Database | DB::Connection | DB::Transaction = PG_DB)
     request = <<-SQL
       DELETE FROM users *
       WHERE email = $1
     SQL
 
-    PG_DB.exec(request, user.email)
+    conn.exec(request, user.email)
   end
 
   # -------------------

src/invidious/routes/account.cr

@@ -124,9 +124,11 @@ module Invidious::Routes::Account
     end
 
     view_name = "subscriptions_#{sha256(user.email)}"
-    Invidious::Database::Users.delete(user)
-    Invidious::Database::SessionIDs.delete(email: user.email)
-    PG_DB.exec("DROP MATERIALIZED VIEW #{view_name}")
+    PG_DB.transaction do |tx|
+      Invidious::Database::Users.delete(user, conn: tx.connection)
+      Invidious::Database::SessionIDs.delete(email: user.email, conn: tx.connection)
+      tx.connection.exec("DROP MATERIALIZED VIEW #{view_name}")
+    end
 
     env.request.cookies.each do |cookie|
       cookie.expires = Time.utc(1990, 1, 1)

src/invidious/routes/login.cr

@@ -122,11 +122,13 @@ module Invidious::Routes::Login
           end
         end
 
-        Invidious::Database::Users.insert(user)
-        Invidious::Database::SessionIDs.insert(sid, email)
+        PG_DB.transaction do |tx|
+          Invidious::Database::Users.insert(user, conn: tx.connection)
+          Invidious::Database::SessionIDs.insert(sid, email, conn: tx.connection)
 
           view_name = "subscriptions_#{sha256(user.email)}"
-        PG_DB.exec("CREATE MATERIALIZED VIEW #{view_name} AS #{MATERIALIZED_VIEW_SQL.call(user.email)}")
+          tx.connection.exec("CREATE MATERIALIZED VIEW #{view_name} AS #{MATERIALIZED_VIEW_SQL.call(user.email)}")
+        end
 
         if alt = CONFIG.alternative_domains.index(host)
           env.response.cookies["SID"] = Invidious::User::Cookies.sid(CONFIG.alternative_domains[alt], sid)