Fix HP demo: emit tag on first line; seeder reconciles demos in place

The reasoning model truncated replies before the end-of-reply [HP-N]
tag, so damage was never reported and HP stuck at 100. Move the tag to
the FIRST line of the reply so it survives truncation, and clarify the
wording (concrete example numbers instead of the literal "[HP-N]" that
confused the model).

Also make the seeder the source of truth for demo content: it now
reconciles an existing seeded scenario in place when a seed file
changes (keeping the scenario row and its adventure FKs), instead of
only inserting when missing — otherwise fixes to already-seeded demos
never reach production. No-op when content already matches, so it stays
cheap on every boot.

Verified end-to-end through the quickjs engine, including a truncated
reply still applying damage, and the in-place update path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TEpWMjnfPqzZ13nPMoGhs5

backend/app/seed_data/03-hp-system-demo.json

@@ -13,7 +13,7 @@
       "description": "AI-driven health. The script tells the AI the current HP and to tag damage/healing as [HP-N]/[HP+N]; the AI decides the amounts, the script applies them to state.hp and hides the tags. Watch state.hp in the left State panel.",
       "library_js": "// Shared HP helpers — available to every hook of this script.\nvar HP_MAX = 100;\nfunction getHp() {\n  if (typeof state.hp !== \"number\") state.hp = HP_MAX;\n  return state.hp;\n}\nfunction setHp(value) {\n  state.hp = Math.max(0, Math.min(HP_MAX, Math.round(value)));\n  return state.hp;\n}\n",
       "input_js": "",
-      "context_js": "// Tell the AI the current HP and how to report changes it decides on.\nvar modifier = function (text) {\n  var hp = getHp();\n  var note =\n    \"\\n\\n[HP SYSTEM] The player currently has \" + hp + \" of \" + HP_MAX + \" HP. \" +\n    \"You, the narrator, decide when and how much the player is hurt or healed based on the story. \" +\n    \"When the player takes damage (an attack, trap, or fall) or heals (a potion, rest, or spell), \" +\n    \"append a tag on its own line at the very end of your reply: [HP-N] for damage or [HP+N] for healing, \" +\n    \"where N is a whole number. For example a solid sword blow might be [HP-14]; a healing potion might be [HP+25]. \" +\n    \"Include the tag only when HP actually changes, and never explain or mention the tag in the prose.\";\n  return { text: text + note };\n};\nmodifier(text);\n",
+      "context_js": "// Tell the AI the current HP and how to report changes it decides on.\n// The tag goes on the FIRST line so it survives even if the reply is cut off.\nvar modifier = function (text) {\n  var hp = getHp();\n  var note =\n    \"\\n\\n[HP SYSTEM] The player currently has \" + hp + \" of \" + HP_MAX + \" health. \" +\n    \"Based on the story, you decide whether the player loses health (an attack, fall, or trap) \" +\n    \"or gains it (a potion, rest, or spell) this turn. \" +\n    \"If their health changes, make the FIRST line of your reply the change by itself in square brackets: \" +\n    \"write [HP-14] to show losing 14, or [HP+25] to show gaining 25 (choose the number that fits the moment). \" +\n    \"Then write the story on the following lines. If health does not change, do not write any bracket tag. \" +\n    \"Never mention this tag or the health system in the story prose itself.\";\n  return { text: text + note };\n};\nmodifier(text);\n",
       "output_js": "// Read the AI's [HP+N]/[HP-N] tags, apply them to state, hide them from the player.\nvar modifier = function (text) {\n  var re = /\\[HP([+-])(\\d{1,3})\\]/g;\n  var hp = getHp();\n  var changed = false;\n  var m;\n  while ((m = re.exec(text)) !== null) {\n    var amount = parseInt(m[2], 10);\n    hp += (m[1] === \"-\" ? -amount : amount);\n    changed = true;\n  }\n  if (changed) setHp(hp);\n  var clean = text\n    .replace(/\\[HP[+-]\\d{1,3}\\]/g, \"\")\n    .replace(/[ \\t]+\\n/g, \"\\n\")\n    .replace(/\\n{3,}/g, \"\\n\\n\")\n    .trim();\n  if (changed && getHp() <= 0 && !state.dead) {\n    state.dead = true;\n    clean += \"\\n\\nYour strength gives out and the world goes dark. (0 HP)\";\n  }\n  return { text: clean };\n};\nmodifier(text);\n"
     }
   ]