Would I theoretically be able to cast this spell on my backpack to let me know if someone is trying to steal from me?

Yes.  And you would want to exclude yourself from tripping the alarm.  And if you cast the spell every 8 hours as a ritual you don't burn any spell slots.

Only if you didn't move your backpack. 

Your video is interesting, but early on you have an error - Casting as a ritual only adds 10 minutes, not a full hour.

https://www.dndbeyond.com/sources/basic-rules/spellcasting

Is it intended to ignore medium sized creatures? Until the spell ends, an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area.

Quote from Dersies >>

Is it intended to ignore medium sized creatures? Until the spell ends, an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area.

 Small, Medium, Large, Huge, and Gargantuan are all examples of creatures larger than Tiny.

No, it doesn't ignore Medium creatures.

Size section of the basic rules.

Are the bell and wire reuseable?

Yes. Spells that consume the components spell it out.

in 5e, there is no size category smaller than tiny, so what creatures could this spell really exclude? would it just be something so small that it does not even have a stat block, like an ant, fly or microscopic organisim?

Quote from ArtificeMeal >>

in 5e, there is no size category smaller than tiny, so what creatures could this spell really exclude? would it just be something so small that it does not even have a stat block, like an ant, fly or microscopic organisim?

 Yes, so the spell doesn’t go off with every single microbe crawling (?) over it.

Nothing in the description text leads me to believe you couldn't put it on an object that was in motion, stopped, and continued to be in motion. Because in the spell description nothing is specific about the location or its relative movement so I would think you'd be able to place it on an object or an area that was in motion ie a cart or literally a backpack on your person. Right?

There's nothing in the rules about how relativity applies to spell areas of effect. It's the DM's call.

You can only cast the spell on a door, window, or area. A backpack is none of these things, and the common meaning of the English word 'area' means a nonmoving area relative to the most salient frame of reference. You may have a DM that lets you get away with using the spell on a moving item, but it's pretty clearly neither RAW or RAI.

Well TheFeyr I'd have to disagree about about it not counting as an area because its the area inside the bag. The area under the bonnet of my car holds the engine, a cream for a rash it will often say apply to the affected area these things are areas relative to objects/people. While yes grammatically the bag is not an area and even the volume of the bag is not an area (area being mathematically being 2D) if you want to be pedantic the opening of a bag definitely. Also non-moving is interesting. what area is your country above water? That is an area in constant flux because of waves and tide or areas. I see nothing against it in RAW but don't know about RAI both there examples are static but Im sure you could make that sort of implied design all over the rules 

Sure, there's enough vagueness in the description to argue in this direction if you really want to. I don't find it at all convincing, but whatever floats your boat in the watery area of your choice. But if you care at all about RAI, you might go slightly beyond the examples given and think about game design. Allowing this 1st level, non-concentration, ritual spell with an 8 hour duration to be cast on objects you carry with you essentially negates the possibility for creatures to approach you undetected while hidden and/or invisible. In fact, depending on how you read 'touches' in the description, it even allows you to actively search for hidden or invisible creatures without even having to use an action. Sure, it won't tell you exactly where they are, but it's still a very useful and powerful ability that you can keep up basically indefinitely at no cost. And this is just one such effect. I'm sure there are others as well. And there are probably other spells that would break as well if you allowed areas to be highly mobile like this.

There a way to make it permanent?

For making it permanent I would follow how permanent teleportation circles work.
Adjust the timing as you'd see fit. Maybe 7 days per X size of wiring.
7 days to cover a doorway, 7 for a window, etc etc.
And probably require it expend a spell slot, not as a ritual.

You can create a permanent teleportation circle by casting this spell in the same location every day for one year. You need not use the circle to teleport when you cast the spell in this way.

All areas are moving. What if you were in a moving cart when you cast it? Then it would be still in relative to the motion of the caster.

So, does the spell end when triggered?

Quote from CadensCavalry >>

All areas are moving.

 Not necessarily; depends how your campaign's universe works.

Quote from CadensCavalry >>

So, does the spell end when triggered?

 It doesn't say so in the text, so no, it lasts the full duration unless dispelled.